diff --git a/packages/typescript/src/ast/ast.generated.ts b/packages/typescript/src/ast/ast.generated.ts
index f4d3a2ca47f83..9c7b5d897b6a9 100644
--- a/packages/typescript/src/ast/ast.generated.ts
+++ b/packages/typescript/src/ast/ast.generated.ts
@@ -754,7 +754,6 @@ export interface SetAccessorDeclaration extends NodeBase, DeclarationBase, Named
 }
 export interface IndexSignatureDeclaration extends NodeBase, DeclarationBase, ModifiersBase, FunctionLikeBase, TypeElementBase, ClassElementBase {
     readonly kind: SyntaxKind.IndexSignature;
-    readonly type: TypeNode;
 }
 export interface MethodSignatureDeclaration extends NodeBase, DeclarationBase, NamedMemberBase, FunctionLikeBase, TypeElementBase {
     readonly kind: SyntaxKind.MethodSignature;
diff --git a/packages/typescript/src/ast/factory.generated.ts b/packages/typescript/src/ast/factory.generated.ts
index 8bc727d7178a1..aeee534813a87 100644
--- a/packages/typescript/src/ast/factory.generated.ts
+++ b/packages/typescript/src/ast/factory.generated.ts
@@ -4357,7 +4357,7 @@ export function createSetAccessorDeclaration(modifiers: readonly ModifierLike[]
     }) as unknown as SetAccessorDeclaration;
 }
 
-export function createIndexSignatureDeclaration(modifiers: readonly ModifierLike[] | undefined, parameters: readonly ParameterDeclaration[], type: TypeNode): IndexSignatureDeclaration {
+export function createIndexSignatureDeclaration(modifiers: readonly ModifierLike[] | undefined, parameters: readonly ParameterDeclaration[], type?: TypeNode): IndexSignatureDeclaration {
     return new NodeObject(SyntaxKind.IndexSignature, {
         modifiers: modifiers ? createNodeArray(modifiers) : undefined,
         parameters: createNodeArray(parameters),
@@ -5591,7 +5591,7 @@ export function updateSetAccessorDeclaration(node: SetAccessorDeclaration, modif
     return node.modifiers !== modifiers || node.name !== name || node.typeParameters !== typeParameters || node.parameters !== parameters || node.type !== type || node.body !== body ? createSetAccessorDeclaration(modifiers, name, typeParameters, parameters, type, body) : node;
 }
 
-export function updateIndexSignatureDeclaration(node: IndexSignatureDeclaration, modifiers: readonly ModifierLike[] | undefined, parameters: readonly ParameterDeclaration[], type: TypeNode): IndexSignatureDeclaration {
+export function updateIndexSignatureDeclaration(node: IndexSignatureDeclaration, modifiers: readonly ModifierLike[] | undefined, parameters: readonly ParameterDeclaration[], type?: TypeNode): IndexSignatureDeclaration {
     return node.modifiers !== modifiers || node.parameters !== parameters || node.type !== type ? createIndexSignatureDeclaration(modifiers, parameters, type) : node;
 }
 
diff --git a/packages/typescript/test/sync/ast.test.ts b/packages/typescript/test/sync/ast.test.ts
index 795965ee5423c..ce86c834401bb 100644
--- a/packages/typescript/test/sync/ast.test.ts
+++ b/packages/typescript/test/sync/ast.test.ts
@@ -54,6 +54,7 @@ import {
     createFunctionDeclaration,
     createIdentifier,
     createIfStatement,
+    createIndexSignatureDeclaration,
     createMissingDeclaration,
     createNodeArray,
     createNumericLiteral,
@@ -108,6 +109,19 @@ describe("NodeObject + childrenIter", { concurrency }, () => {
     });
 });
 
+test("index signatures allow an omitted type", () => {
+    using api = spawnAPI();
+    const source = api.createSourceFile("/repro.ts", "interface I { [key: string]; next: number; }").sourceFile;
+    const declaration = source.statements[0];
+    assert.ok(astModule.isInterfaceDeclaration(declaration));
+    const signature = declaration.members[0];
+    assert.ok(astModule.isIndexSignatureDeclaration(signature));
+    const type: astModule.TypeNode | undefined = signature.type;
+    assert.strictEqual(type, undefined);
+    assert.strictEqual(createIndexSignatureDeclaration(undefined, signature.parameters).type, undefined);
+    assert.strictEqual(declaration.members.length, 2);
+});
+
 describe("AST property contracts", { concurrency }, () => {
     test("absent parser children have optional types", () => {
         const absent: {
diff --git a/tools/scripts/tsc/ast.json b/tools/scripts/tsc/ast.json
index 83d1e1fa305fb..b79180fd41fd2 100644
--- a/tools/scripts/tsc/ast.json
+++ b/tools/scripts/tsc/ast.json
@@ -2497,7 +2497,7 @@
                     {
                         "name": "Type",
                         "inherited": true,
-                        "optional": false
+                        "optional": true
                     }
                 ]
             },
diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index bdedc411bdb9c..53621ed093cee 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -2834,6 +2834,8 @@ func (c *Checker) checkSignatureDeclaration(node *ast.Node) {
 			c.error(node, diagnostics.Construct_signature_which_lacks_return_type_annotation_implicitly_has_an_any_return_type)
 		case ast.KindCallSignature:
 			c.error(node, diagnostics.Call_signature_which_lacks_return_type_annotation_implicitly_has_an_any_return_type)
+		case ast.KindIndexSignature:
+			c.error(node, diagnostics.Index_signature_which_lacks_type_annotation_implicitly_has_an_any_type)
 		}
 	}
 	if returnTypeNode != nil {
diff --git a/tsc/internal/checker/grammarchecks.go b/tsc/internal/checker/grammarchecks.go
index 93bed4b355296..f8997b8e361e1 100644
--- a/tsc/internal/checker/grammarchecks.go
+++ b/tsc/internal/checker/grammarchecks.go
@@ -831,9 +831,6 @@ func (c *Checker) checkGrammarIndexSignatureParameters(node *ast.IndexSignatureD
 	if !everyType(t, c.isValidIndexKeyType) {
 		return c.grammarErrorOnNode(parameter.Name(), diagnostics.An_index_signature_parameter_type_must_be_string_number_symbol_or_a_template_literal_type)
 	}
-	if node.Type == nil {
-		return c.grammarErrorOnNode(node.AsNode(), diagnostics.An_index_signature_must_have_a_type_annotation)
-	}
 	return false
 }
 
diff --git a/tsc/internal/diagnostics/diagnosticMessages.generated.json b/tsc/internal/diagnostics/diagnosticMessages.generated.json
index ae749e01b85b5..394005f1c22b6 100644
--- a/tsc/internal/diagnostics/diagnosticMessages.generated.json
+++ b/tsc/internal/diagnostics/diagnosticMessages.generated.json
@@ -16,7 +16,6 @@
     "An_index_signature_parameter_cannot_have_an_accessibility_modifier_1018": "An index signature parameter cannot have an accessibility modifier.",
     "An_index_signature_parameter_cannot_have_a_question_mark_1019": "An index signature parameter cannot have a question mark.",
     "An_index_signature_parameter_cannot_have_an_initializer_1020": "An index signature parameter cannot have an initializer.",
-    "An_index_signature_must_have_a_type_annotation_1021": "An index signature must have a type annotation.",
     "An_index_signature_parameter_must_have_a_type_annotation_1022": "An index signature parameter must have a type annotation.",
     "readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature_1024": "'readonly' modifier can only appear on a property declaration or index signature.",
     "An_index_signature_cannot_have_a_trailing_comma_1025": "An index signature cannot have a trailing comma.",
@@ -1751,6 +1750,7 @@
     "This_syntax_is_reserved_in_files_with_the_mts_or_cts_extension_Use_an_as_expression_instead_7059": "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead.",
     "This_syntax_is_reserved_in_files_with_the_mts_or_cts_extension_Add_a_trailing_comma_or_explicit_cons_7060": "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma or explicit constraint.",
     "A_mapped_type_may_not_declare_properties_or_methods_7061": "A mapped type may not declare properties or methods.",
+    "Index_signature_which_lacks_type_annotation_implicitly_has_an_any_type_7062": "Index signature, which lacks type annotation, implicitly has an 'any' type.",
     "Declaration_emit_elides_private_members_but_0_refers_to_a_private_member_Write_an_explicit_type_here_7080": "Declaration emit elides private members, but '{0}' refers to a private member. Write an explicit type here.",
     "You_cannot_rename_this_element_8000": "You cannot rename this element.",
     "You_cannot_rename_elements_that_are_defined_in_the_standard_TypeScript_library_8001": "You cannot rename elements that are defined in the standard TypeScript library.",
diff --git a/tsc/internal/diagnostics/diagnosticMessages.json b/tsc/internal/diagnostics/diagnosticMessages.json
index d4bf59ab839de..192ecc22f145d 100644
--- a/tsc/internal/diagnostics/diagnosticMessages.json
+++ b/tsc/internal/diagnostics/diagnosticMessages.json
@@ -67,10 +67,6 @@
         "category": "Error",
         "code": 1020
     },
-    "An index signature must have a type annotation.": {
-        "category": "Error",
-        "code": 1021
-    },
     "An index signature parameter must have a type annotation.": {
         "category": "Error",
         "code": 1022
@@ -7042,6 +7038,10 @@
         "category": "Error",
         "code": 7061
     },
+    "Index signature, which lacks type annotation, implicitly has an 'any' type.": {
+        "category": "Error",
+        "code": 7062
+    },
     "Declaration emit elides private members, but '{0}' refers to a private member. Write an explicit type here.": {
         "category": "Error",
         "code": 7080
diff --git a/tsc/internal/diagnostics/diagnostics_generated.go b/tsc/internal/diagnostics/diagnostics_generated.go
index 0e08cf23701b7..3c9854f8ecb01 100644
--- a/tsc/internal/diagnostics/diagnostics_generated.go
+++ b/tsc/internal/diagnostics/diagnostics_generated.go
@@ -36,8 +36,6 @@ var An_index_signature_parameter_cannot_have_a_question_mark = &Message{code: 10
 
 var An_index_signature_parameter_cannot_have_an_initializer = &Message{code: 1020, category: CategoryError, key: "An_index_signature_parameter_cannot_have_an_initializer_1020", text: "An index signature parameter cannot have an initializer."}
 
-var An_index_signature_must_have_a_type_annotation = &Message{code: 1021, category: CategoryError, key: "An_index_signature_must_have_a_type_annotation_1021", text: "An index signature must have a type annotation."}
-
 var An_index_signature_parameter_must_have_a_type_annotation = &Message{code: 1022, category: CategoryError, key: "An_index_signature_parameter_must_have_a_type_annotation_1022", text: "An index signature parameter must have a type annotation."}
 
 var X_readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature = &Message{code: 1024, category: CategoryError, key: "readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature_1024", text: "'readonly' modifier can only appear on a property declaration or index signature."}
@@ -3506,6 +3504,8 @@ var This_syntax_is_reserved_in_files_with_the_mts_or_cts_extension_Add_a_trailin
 
 var A_mapped_type_may_not_declare_properties_or_methods = &Message{code: 7061, category: CategoryError, key: "A_mapped_type_may_not_declare_properties_or_methods_7061", text: "A mapped type may not declare properties or methods."}
 
+var Index_signature_which_lacks_type_annotation_implicitly_has_an_any_type = &Message{code: 7062, category: CategoryError, key: "Index_signature_which_lacks_type_annotation_implicitly_has_an_any_type_7062", text: "Index signature, which lacks type annotation, implicitly has an 'any' type."}
+
 var Declaration_emit_elides_private_members_but_0_refers_to_a_private_member_Write_an_explicit_type_here = &Message{code: 7080, category: CategoryError, key: "Declaration_emit_elides_private_members_but_0_refers_to_a_private_member_Write_an_explicit_type_here_7080", text: "Declaration emit elides private members, but '{0}' refers to a private member. Write an explicit type here."}
 
 var You_cannot_rename_this_element = &Message{code: 8000, category: CategoryError, key: "You_cannot_rename_this_element_8000", text: "You cannot rename this element."}
@@ -4480,7 +4480,6 @@ var allMessages = [...]**Message{
 	&An_index_signature_parameter_cannot_have_an_accessibility_modifier,
 	&An_index_signature_parameter_cannot_have_a_question_mark,
 	&An_index_signature_parameter_cannot_have_an_initializer,
-	&An_index_signature_must_have_a_type_annotation,
 	&An_index_signature_parameter_must_have_a_type_annotation,
 	&X_readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature,
 	&An_index_signature_cannot_have_a_trailing_comma,
@@ -6215,6 +6214,7 @@ var allMessages = [...]**Message{
 	&This_syntax_is_reserved_in_files_with_the_mts_or_cts_extension_Use_an_as_expression_instead,
 	&This_syntax_is_reserved_in_files_with_the_mts_or_cts_extension_Add_a_trailing_comma_or_explicit_constraint,
 	&A_mapped_type_may_not_declare_properties_or_methods,
+	&Index_signature_which_lacks_type_annotation_implicitly_has_an_any_type,
 	&Declaration_emit_elides_private_members_but_0_refers_to_a_private_member_Write_an_explicit_type_here,
 	&You_cannot_rename_this_element,
 	&You_cannot_rename_elements_that_are_defined_in_the_standard_TypeScript_library,
diff --git a/tsc/internal/diagnostics/loc/cs-CZ.json.gz b/tsc/internal/diagnostics/loc/cs-CZ.json.gz
index e7fc4cd38bae8..5a8a2f66e3120 100644
Binary files a/tsc/internal/diagnostics/loc/cs-CZ.json.gz and b/tsc/internal/diagnostics/loc/cs-CZ.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/de-DE.json.gz b/tsc/internal/diagnostics/loc/de-DE.json.gz
index 7ca0434996b49..cc94cca803f58 100644
Binary files a/tsc/internal/diagnostics/loc/de-DE.json.gz and b/tsc/internal/diagnostics/loc/de-DE.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/es-ES.json.gz b/tsc/internal/diagnostics/loc/es-ES.json.gz
index d856bcbcbc4d1..1890e00931de7 100644
Binary files a/tsc/internal/diagnostics/loc/es-ES.json.gz and b/tsc/internal/diagnostics/loc/es-ES.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/fr-FR.json.gz b/tsc/internal/diagnostics/loc/fr-FR.json.gz
index 1b55112a59e26..d5ed3afd4c45d 100644
Binary files a/tsc/internal/diagnostics/loc/fr-FR.json.gz and b/tsc/internal/diagnostics/loc/fr-FR.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/it-IT.json.gz b/tsc/internal/diagnostics/loc/it-IT.json.gz
index 8366cae3ef2e5..d512180a60ed7 100644
Binary files a/tsc/internal/diagnostics/loc/it-IT.json.gz and b/tsc/internal/diagnostics/loc/it-IT.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/ja-JP.json.gz b/tsc/internal/diagnostics/loc/ja-JP.json.gz
index 83462c0e5a8d1..df30dedcf4836 100644
Binary files a/tsc/internal/diagnostics/loc/ja-JP.json.gz and b/tsc/internal/diagnostics/loc/ja-JP.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/ko-KR.json.gz b/tsc/internal/diagnostics/loc/ko-KR.json.gz
index 0c46e16e45a07..e54c24733c932 100644
Binary files a/tsc/internal/diagnostics/loc/ko-KR.json.gz and b/tsc/internal/diagnostics/loc/ko-KR.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/pl-PL.json.gz b/tsc/internal/diagnostics/loc/pl-PL.json.gz
index 36ccf0e7a1c7e..79cecff3e5340 100644
Binary files a/tsc/internal/diagnostics/loc/pl-PL.json.gz and b/tsc/internal/diagnostics/loc/pl-PL.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/pt-BR.json.gz b/tsc/internal/diagnostics/loc/pt-BR.json.gz
index 9f1dbef5d4066..a056abdd16a52 100644
Binary files a/tsc/internal/diagnostics/loc/pt-BR.json.gz and b/tsc/internal/diagnostics/loc/pt-BR.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/ru-RU.json.gz b/tsc/internal/diagnostics/loc/ru-RU.json.gz
index b22cdcce3c625..e0f9bbb684fc2 100644
Binary files a/tsc/internal/diagnostics/loc/ru-RU.json.gz and b/tsc/internal/diagnostics/loc/ru-RU.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/tr-TR.json.gz b/tsc/internal/diagnostics/loc/tr-TR.json.gz
index 0bf72b0d65166..b08cebd3c5633 100644
Binary files a/tsc/internal/diagnostics/loc/tr-TR.json.gz and b/tsc/internal/diagnostics/loc/tr-TR.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/zh-CN.json.gz b/tsc/internal/diagnostics/loc/zh-CN.json.gz
index 90902d6a434e3..24ff3aebed33f 100644
Binary files a/tsc/internal/diagnostics/loc/zh-CN.json.gz and b/tsc/internal/diagnostics/loc/zh-CN.json.gz differ
diff --git a/tsc/internal/diagnostics/loc/zh-TW.json.gz b/tsc/internal/diagnostics/loc/zh-TW.json.gz
index 2291815290199..e583b3b0b0450 100644
Binary files a/tsc/internal/diagnostics/loc/zh-TW.json.gz and b/tsc/internal/diagnostics/loc/zh-TW.json.gz differ
diff --git a/tsc/testdata/baselines/reference/compiler/arraySigChecking.errors.txt b/tsc/testdata/baselines/reference/compiler/arraySigChecking.errors.txt
index 74dbd3487bfe2..603b706b5c3f7 100644
--- a/tsc/testdata/baselines/reference/compiler/arraySigChecking.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/arraySigChecking.errors.txt
@@ -1,10 +1,11 @@
+arraySigChecking.ts(11,16): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 arraySigChecking.ts(11,17): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
 arraySigChecking.ts(18,27): error TS2322: Type 'void' is not assignable to type 'string'.
 arraySigChecking.ts(22,13): error TS2322: Type 'number' is not assignable to type 'number[]'.
 arraySigChecking.ts(22,16): error TS2322: Type 'number' is not assignable to type 'number[]'.
 
 
-==== arraySigChecking.ts (4 errors) ====
+==== arraySigChecking.ts (5 errors) ====
     declare namespace M {
         interface iBar { t: any; }
         interface iFoo extends iBar {
@@ -16,6 +17,8 @@ arraySigChecking.ts(22,16): error TS2322: Type 'number' is not assignable to typ
         }
     
         var foo: { [index: any]; }; // expect an error here
+                   ~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
                     ~~~~~
 !!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
     }
diff --git a/tsc/testdata/baselines/reference/compiler/giant.errors.txt b/tsc/testdata/baselines/reference/compiler/giant.errors.txt
index 248efb5e0f8a7..4768a09ada236 100644
--- a/tsc/testdata/baselines/reference/compiler/giant.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/giant.errors.txt
@@ -16,7 +16,6 @@ giant.ts(34,29): error TS1005: '{' expected.
 giant.ts(35,12): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(36,16): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(36,20): error TS1005: '{' expected.
-giant.ts(62,5): error TS1021: An index signature must have a type annotation.
 giant.ts(63,6): error TS1096: An index signature must have exactly one parameter.
 giant.ts(76,5): error TS2386: Overload signatures must all be optional or required.
 giant.ts(87,16): error TS2300: Duplicate identifier 'pgF'.
@@ -37,7 +36,6 @@ giant.ts(98,33): error TS1005: '{' expected.
 giant.ts(99,16): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(100,20): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(100,24): error TS1005: '{' expected.
-giant.ts(126,9): error TS1021: An index signature must have a type annotation.
 giant.ts(127,10): error TS1096: An index signature must have exactly one parameter.
 giant.ts(140,9): error TS2386: Overload signatures must all be optional or required.
 giant.ts(154,39): error TS1183: An implementation cannot be declared in ambient contexts.
@@ -59,7 +57,6 @@ giant.ts(177,33): error TS1005: '{' expected.
 giant.ts(178,16): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(179,20): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(179,24): error TS1005: '{' expected.
-giant.ts(205,9): error TS1021: An index signature must have a type annotation.
 giant.ts(206,10): error TS1096: An index signature must have exactly one parameter.
 giant.ts(219,9): error TS2386: Overload signatures must all be optional or required.
 giant.ts(233,39): error TS1183: An implementation cannot be declared in ambient contexts.
@@ -107,7 +104,6 @@ giant.ts(292,29): error TS1005: '{' expected.
 giant.ts(293,12): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(294,16): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(294,20): error TS1005: '{' expected.
-giant.ts(320,5): error TS1021: An index signature must have a type annotation.
 giant.ts(321,6): error TS1096: An index signature must have exactly one parameter.
 giant.ts(334,5): error TS2386: Overload signatures must all be optional or required.
 giant.ts(345,16): error TS2300: Duplicate identifier 'pgF'.
@@ -128,7 +124,6 @@ giant.ts(356,33): error TS1005: '{' expected.
 giant.ts(357,16): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(358,20): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(358,24): error TS1005: '{' expected.
-giant.ts(384,9): error TS1021: An index signature must have a type annotation.
 giant.ts(385,10): error TS1096: An index signature must have exactly one parameter.
 giant.ts(398,9): error TS2386: Overload signatures must all be optional or required.
 giant.ts(412,39): error TS1183: An implementation cannot be declared in ambient contexts.
@@ -150,7 +145,6 @@ giant.ts(435,33): error TS1005: '{' expected.
 giant.ts(436,16): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(437,20): error TS2300: Duplicate identifier 'tgF'.
 giant.ts(437,24): error TS1005: '{' expected.
-giant.ts(463,9): error TS1021: An index signature must have a type annotation.
 giant.ts(464,10): error TS1096: An index signature must have exactly one parameter.
 giant.ts(477,9): error TS2386: Overload signatures must all be optional or required.
 giant.ts(491,39): error TS1183: An implementation cannot be declared in ambient contexts.
@@ -208,7 +202,6 @@ giant.ts(556,21): error TS1036: Statements are not allowed in ambient contexts.
 giant.ts(558,24): error TS1183: An implementation cannot be declared in ambient contexts.
 giant.ts(561,21): error TS1183: An implementation cannot be declared in ambient contexts.
 giant.ts(563,21): error TS1183: An implementation cannot be declared in ambient contexts.
-giant.ts(588,9): error TS1021: An index signature must have a type annotation.
 giant.ts(589,10): error TS1096: An index signature must have exactly one parameter.
 giant.ts(602,9): error TS2386: Overload signatures must all be optional or required.
 giant.ts(606,22): error TS1183: An implementation cannot be declared in ambient contexts.
@@ -223,7 +216,6 @@ giant.ts(621,26): error TS1183: An implementation cannot be declared in ambient
 giant.ts(623,24): error TS1183: An implementation cannot be declared in ambient contexts.
 giant.ts(626,21): error TS1183: An implementation cannot be declared in ambient contexts.
 giant.ts(628,21): error TS1183: An implementation cannot be declared in ambient contexts.
-giant.ts(654,9): error TS1021: An index signature must have a type annotation.
 giant.ts(655,10): error TS1096: An index signature must have exactly one parameter.
 giant.ts(668,9): error TS2386: Overload signatures must all be optional or required.
 giant.ts(672,22): error TS1183: An implementation cannot be declared in ambient contexts.
@@ -231,7 +223,7 @@ giant.ts(672,25): error TS1036: Statements are not allowed in ambient contexts.
 giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient contexts.
 
 
-==== giant.ts (231 errors) ====
+==== giant.ts (223 errors) ====
     /*
         Prefixes
         p -> public
@@ -330,8 +322,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
         //Index Signature
         [p];
         [p1: string];
-        ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
         [p2: string, p3: number];
          ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -436,8 +426,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
             //Index Signature
             [p];
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -559,8 +547,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
             //Index Signature
             [p];
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -770,8 +756,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
         //Index Signature
         [p];
         [p1: string];
-        ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
         [p2: string, p3: number];
          ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -876,8 +860,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
             //Index Signature
             [p];
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -999,8 +981,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
             //Index Signature
             [p];
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -1240,8 +1220,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
             //Index Signature
             [p];
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
@@ -1336,8 +1314,6 @@ giant.ts(676,30): error TS1183: An implementation cannot be declared in ambient
             //Index Signature
             [p];
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).errors.txt b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).errors.txt
new file mode 100644
index 0000000000000..86a0036e74609
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).errors.txt
@@ -0,0 +1,74 @@
+indexSignatureMissingType.ts(33,1): error TS2542: Index signature in type 'C' only permits reading.
+indexSignatureMissingType.ts(36,9): error TS1019: An index signature parameter cannot have a question mark.
+indexSignatureMissingType.ts(40,6): error TS1017: An index signature cannot have a rest parameter.
+indexSignatureMissingType.ts(44,6): error TS1096: An index signature must have exactly one parameter.
+indexSignatureMissingType.ts(48,6): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
+
+
+==== indexSignatureMissingType.ts (5 errors) ====
+    export interface I {
+        [key: string];
+    }
+    
+    export class C {
+        readonly [key: number];
+    }
+    
+    export type T = {
+        [key: string]
+        property: number;
+    };
+    
+    export interface NumberIndex { [key: number]; }
+    export interface SymbolIndex { [key: symbol]; }
+    export interface TemplateIndex { [key: `prefix-${string}`]; }
+    export interface ExplicitAny { [key: string]: any; }
+    export interface ExplicitUnknown { [key: string]: unknown; }
+    
+    declare const value: I;
+    export const asNumber: number = value.anyKey;
+    export const asString: string = value.anyKey;
+    export const indexedValue = value.anyKey;
+    
+    declare const instance: C;
+    declare const numbers: NumberIndex;
+    declare const symbols: SymbolIndex;
+    declare const templates: TemplateIndex;
+    export const classValue = instance[0];
+    export const numberValue = numbers[0];
+    export const symbolValue = symbols[Symbol.iterator];
+    export const templateValue = templates["prefix-foo"];
+    instance[0] = 1;
+    ~~~~~~~~~~~
+!!! error TS2542: Index signature in type 'C' only permits reading.
+    
+    interface OptionalParameter {
+        [key?: string];
+            ~
+!!! error TS1019: An index signature parameter cannot have a question mark.
+    }
+    
+    interface RestParameter {
+        [...key: string[]];
+         ~~~
+!!! error TS1017: An index signature cannot have a rest parameter.
+    }
+    
+    interface MultipleParameters {
+        [key: string, other: number];
+         ~~~
+!!! error TS1096: An index signature must have exactly one parameter.
+    }
+    
+    interface InvalidKey {
+        [key: boolean];
+         ~~~
+!!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
+    }
+    
+    // @ts-ignore
+    interface Ignored { [key: string]; }
+    
+    // @ts-expect-error
+    interface Expected { [key: boolean]; }
+    
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).js b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).js
new file mode 100644
index 0000000000000..4cbf08f85a90c
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).js
@@ -0,0 +1,106 @@
+//// [tests/cases/compiler/indexSignatureMissingType.ts] ////
+
+//// [indexSignatureMissingType.ts]
+export interface I {
+    [key: string];
+}
+
+export class C {
+    readonly [key: number];
+}
+
+export type T = {
+    [key: string]
+    property: number;
+};
+
+export interface NumberIndex { [key: number]; }
+export interface SymbolIndex { [key: symbol]; }
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+export interface ExplicitAny { [key: string]: any; }
+export interface ExplicitUnknown { [key: string]: unknown; }
+
+declare const value: I;
+export const asNumber: number = value.anyKey;
+export const asString: string = value.anyKey;
+export const indexedValue = value.anyKey;
+
+declare const instance: C;
+declare const numbers: NumberIndex;
+declare const symbols: SymbolIndex;
+declare const templates: TemplateIndex;
+export const classValue = instance[0];
+export const numberValue = numbers[0];
+export const symbolValue = symbols[Symbol.iterator];
+export const templateValue = templates["prefix-foo"];
+instance[0] = 1;
+
+interface OptionalParameter {
+    [key?: string];
+}
+
+interface RestParameter {
+    [...key: string[]];
+}
+
+interface MultipleParameters {
+    [key: string, other: number];
+}
+
+interface InvalidKey {
+    [key: boolean];
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }
+
+
+//// [indexSignatureMissingType.js]
+export class C {
+}
+export const asNumber = value.anyKey;
+export const asString = value.anyKey;
+export const indexedValue = value.anyKey;
+export const classValue = instance[0];
+export const numberValue = numbers[0];
+export const symbolValue = symbols[Symbol.iterator];
+export const templateValue = templates["prefix-foo"];
+instance[0] = 1;
+
+
+//// [indexSignatureMissingType.d.ts]
+export interface I {
+    [key: string]: any;
+}
+export declare class C {
+    readonly [key: number]: any;
+}
+export type T = {
+    [key: string]: any;
+    property: number;
+};
+export interface NumberIndex {
+    [key: number]: any;
+}
+export interface SymbolIndex {
+    [key: symbol]: any;
+}
+export interface TemplateIndex {
+    [key: `prefix-${string}`]: any;
+}
+export interface ExplicitAny {
+    [key: string]: any;
+}
+export interface ExplicitUnknown {
+    [key: string]: unknown;
+}
+export declare const asNumber: number;
+export declare const asString: string;
+export declare const indexedValue: any;
+export declare const classValue: any;
+export declare const numberValue: any;
+export declare const symbolValue: any;
+export declare const templateValue: any;
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).symbols b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).symbols
new file mode 100644
index 0000000000000..8897bd254269d
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).symbols
@@ -0,0 +1,147 @@
+//// [tests/cases/compiler/indexSignatureMissingType.ts] ////
+
+=== indexSignatureMissingType.ts ===
+export interface I {
+>I : Symbol(I, Decl(indexSignatureMissingType.ts, 0, 0))
+
+    [key: string];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 1, 5))
+}
+
+export class C {
+>C : Symbol(C, Decl(indexSignatureMissingType.ts, 2, 1))
+
+    readonly [key: number];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 5, 14))
+}
+
+export type T = {
+>T : Symbol(T, Decl(indexSignatureMissingType.ts, 6, 1))
+
+    [key: string]
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 9, 5))
+
+    property: number;
+>property : Symbol(property, Decl(indexSignatureMissingType.ts, 9, 17))
+
+};
+
+export interface NumberIndex { [key: number]; }
+>NumberIndex : Symbol(NumberIndex, Decl(indexSignatureMissingType.ts, 11, 2))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 13, 32))
+
+export interface SymbolIndex { [key: symbol]; }
+>SymbolIndex : Symbol(SymbolIndex, Decl(indexSignatureMissingType.ts, 13, 47))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 14, 32))
+
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+>TemplateIndex : Symbol(TemplateIndex, Decl(indexSignatureMissingType.ts, 14, 47))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 15, 34))
+
+export interface ExplicitAny { [key: string]: any; }
+>ExplicitAny : Symbol(ExplicitAny, Decl(indexSignatureMissingType.ts, 15, 61))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 16, 32))
+
+export interface ExplicitUnknown { [key: string]: unknown; }
+>ExplicitUnknown : Symbol(ExplicitUnknown, Decl(indexSignatureMissingType.ts, 16, 52))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 17, 36))
+
+declare const value: I;
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>I : Symbol(I, Decl(indexSignatureMissingType.ts, 0, 0))
+
+export const asNumber: number = value.anyKey;
+>asNumber : Symbol(asNumber, Decl(indexSignatureMissingType.ts, 20, 12))
+>value.anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+
+export const asString: string = value.anyKey;
+>asString : Symbol(asString, Decl(indexSignatureMissingType.ts, 21, 12))
+>value.anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+
+export const indexedValue = value.anyKey;
+>indexedValue : Symbol(indexedValue, Decl(indexSignatureMissingType.ts, 22, 12))
+>value.anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+
+declare const instance: C;
+>instance : Symbol(instance, Decl(indexSignatureMissingType.ts, 24, 13))
+>C : Symbol(C, Decl(indexSignatureMissingType.ts, 2, 1))
+
+declare const numbers: NumberIndex;
+>numbers : Symbol(numbers, Decl(indexSignatureMissingType.ts, 25, 13))
+>NumberIndex : Symbol(NumberIndex, Decl(indexSignatureMissingType.ts, 11, 2))
+
+declare const symbols: SymbolIndex;
+>symbols : Symbol(symbols, Decl(indexSignatureMissingType.ts, 26, 13))
+>SymbolIndex : Symbol(SymbolIndex, Decl(indexSignatureMissingType.ts, 13, 47))
+
+declare const templates: TemplateIndex;
+>templates : Symbol(templates, Decl(indexSignatureMissingType.ts, 27, 13))
+>TemplateIndex : Symbol(TemplateIndex, Decl(indexSignatureMissingType.ts, 14, 47))
+
+export const classValue = instance[0];
+>classValue : Symbol(classValue, Decl(indexSignatureMissingType.ts, 28, 12))
+>instance : Symbol(instance, Decl(indexSignatureMissingType.ts, 24, 13))
+
+export const numberValue = numbers[0];
+>numberValue : Symbol(numberValue, Decl(indexSignatureMissingType.ts, 29, 12))
+>numbers : Symbol(numbers, Decl(indexSignatureMissingType.ts, 25, 13))
+
+export const symbolValue = symbols[Symbol.iterator];
+>symbolValue : Symbol(symbolValue, Decl(indexSignatureMissingType.ts, 30, 12))
+>symbols : Symbol(symbols, Decl(indexSignatureMissingType.ts, 26, 13))
+>Symbol.iterator : Symbol(SymbolConstructor.iterator, Decl(lib.es2015.iterable.d.ts, --, --))
+>Symbol : Symbol(Symbol, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.symbol.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2019.symbol.d.ts, --, --))
+>iterator : Symbol(SymbolConstructor.iterator, Decl(lib.es2015.iterable.d.ts, --, --))
+
+export const templateValue = templates["prefix-foo"];
+>templateValue : Symbol(templateValue, Decl(indexSignatureMissingType.ts, 31, 12))
+>templates : Symbol(templates, Decl(indexSignatureMissingType.ts, 27, 13))
+
+instance[0] = 1;
+>instance : Symbol(instance, Decl(indexSignatureMissingType.ts, 24, 13))
+
+interface OptionalParameter {
+>OptionalParameter : Symbol(OptionalParameter, Decl(indexSignatureMissingType.ts, 32, 16))
+
+    [key?: string];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 35, 5))
+}
+
+interface RestParameter {
+>RestParameter : Symbol(RestParameter, Decl(indexSignatureMissingType.ts, 36, 1))
+
+    [...key: string[]];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 39, 5))
+}
+
+interface MultipleParameters {
+>MultipleParameters : Symbol(MultipleParameters, Decl(indexSignatureMissingType.ts, 40, 1))
+
+    [key: string, other: number];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 43, 5))
+>other : Symbol(other, Decl(indexSignatureMissingType.ts, 43, 17))
+}
+
+interface InvalidKey {
+>InvalidKey : Symbol(InvalidKey, Decl(indexSignatureMissingType.ts, 44, 1))
+
+    [key: boolean];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 47, 5))
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+>Ignored : Symbol(Ignored, Decl(indexSignatureMissingType.ts, 48, 1))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 51, 21))
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }
+>Expected : Symbol(Expected, Decl(indexSignatureMissingType.ts, 51, 36))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 54, 22))
+
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).types b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).types
new file mode 100644
index 0000000000000..5d3d1bcdb256b
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=false).types
@@ -0,0 +1,136 @@
+//// [tests/cases/compiler/indexSignatureMissingType.ts] ////
+
+=== indexSignatureMissingType.ts ===
+export interface I {
+    [key: string];
+>key : string
+}
+
+export class C {
+>C : C
+
+    readonly [key: number];
+>key : number
+}
+
+export type T = {
+>T : T
+
+    [key: string]
+>key : string
+
+    property: number;
+>property : number
+
+};
+
+export interface NumberIndex { [key: number]; }
+>key : number
+
+export interface SymbolIndex { [key: symbol]; }
+>key : symbol
+
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+>key : `prefix-${string}`
+
+export interface ExplicitAny { [key: string]: any; }
+>key : string
+
+export interface ExplicitUnknown { [key: string]: unknown; }
+>key : string
+
+declare const value: I;
+>value : I
+
+export const asNumber: number = value.anyKey;
+>asNumber : number
+>value.anyKey : any
+>value : I
+>anyKey : any
+
+export const asString: string = value.anyKey;
+>asString : string
+>value.anyKey : any
+>value : I
+>anyKey : any
+
+export const indexedValue = value.anyKey;
+>indexedValue : any
+>value.anyKey : any
+>value : I
+>anyKey : any
+
+declare const instance: C;
+>instance : C
+
+declare const numbers: NumberIndex;
+>numbers : NumberIndex
+
+declare const symbols: SymbolIndex;
+>symbols : SymbolIndex
+
+declare const templates: TemplateIndex;
+>templates : TemplateIndex
+
+export const classValue = instance[0];
+>classValue : any
+>instance[0] : any
+>instance : C
+>0 : 0
+
+export const numberValue = numbers[0];
+>numberValue : any
+>numbers[0] : any
+>numbers : NumberIndex
+>0 : 0
+
+export const symbolValue = symbols[Symbol.iterator];
+>symbolValue : any
+>symbols[Symbol.iterator] : any
+>symbols : SymbolIndex
+>Symbol.iterator : unique symbol
+>Symbol : SymbolConstructor
+>iterator : unique symbol
+
+export const templateValue = templates["prefix-foo"];
+>templateValue : any
+>templates["prefix-foo"] : any
+>templates : TemplateIndex
+>"prefix-foo" : "prefix-foo"
+
+instance[0] = 1;
+>instance[0] = 1 : 1
+>instance[0] : any
+>instance : C
+>0 : 0
+>1 : 1
+
+interface OptionalParameter {
+    [key?: string];
+>key : string | undefined
+}
+
+interface RestParameter {
+    [...key: string[]];
+>key : string[]
+}
+
+interface MultipleParameters {
+    [key: string, other: number];
+>key : string
+>other : number
+}
+
+interface InvalidKey {
+    [key: boolean];
+>key : boolean
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+>key : string
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }
+>key : boolean
+
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).errors.txt b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).errors.txt
new file mode 100644
index 0000000000000..5e3a9f67445ef
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).errors.txt
@@ -0,0 +1,104 @@
+indexSignatureMissingType.ts(2,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(6,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(10,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(14,32): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(15,32): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(16,34): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(33,1): error TS2542: Index signature in type 'C' only permits reading.
+indexSignatureMissingType.ts(36,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(36,9): error TS1019: An index signature parameter cannot have a question mark.
+indexSignatureMissingType.ts(40,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(40,6): error TS1017: An index signature cannot have a rest parameter.
+indexSignatureMissingType.ts(44,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(44,6): error TS1096: An index signature must have exactly one parameter.
+indexSignatureMissingType.ts(48,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+indexSignatureMissingType.ts(48,6): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
+
+
+==== indexSignatureMissingType.ts (15 errors) ====
+    export interface I {
+        [key: string];
+        ~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+    }
+    
+    export class C {
+        readonly [key: number];
+        ~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+    }
+    
+    export type T = {
+        [key: string]
+        ~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+        property: number;
+    };
+    
+    export interface NumberIndex { [key: number]; }
+                                   ~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+    export interface SymbolIndex { [key: symbol]; }
+                                   ~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+    export interface TemplateIndex { [key: `prefix-${string}`]; }
+                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+    export interface ExplicitAny { [key: string]: any; }
+    export interface ExplicitUnknown { [key: string]: unknown; }
+    
+    declare const value: I;
+    export const asNumber: number = value.anyKey;
+    export const asString: string = value.anyKey;
+    export const indexedValue = value.anyKey;
+    
+    declare const instance: C;
+    declare const numbers: NumberIndex;
+    declare const symbols: SymbolIndex;
+    declare const templates: TemplateIndex;
+    export const classValue = instance[0];
+    export const numberValue = numbers[0];
+    export const symbolValue = symbols[Symbol.iterator];
+    export const templateValue = templates["prefix-foo"];
+    instance[0] = 1;
+    ~~~~~~~~~~~
+!!! error TS2542: Index signature in type 'C' only permits reading.
+    
+    interface OptionalParameter {
+        [key?: string];
+        ~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+            ~
+!!! error TS1019: An index signature parameter cannot have a question mark.
+    }
+    
+    interface RestParameter {
+        [...key: string[]];
+        ~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+         ~~~
+!!! error TS1017: An index signature cannot have a rest parameter.
+    }
+    
+    interface MultipleParameters {
+        [key: string, other: number];
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+         ~~~
+!!! error TS1096: An index signature must have exactly one parameter.
+    }
+    
+    interface InvalidKey {
+        [key: boolean];
+        ~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
+         ~~~
+!!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
+    }
+    
+    // @ts-ignore
+    interface Ignored { [key: string]; }
+    
+    // @ts-expect-error
+    interface Expected { [key: boolean]; }
+    
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).js b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).js
new file mode 100644
index 0000000000000..4cbf08f85a90c
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).js
@@ -0,0 +1,106 @@
+//// [tests/cases/compiler/indexSignatureMissingType.ts] ////
+
+//// [indexSignatureMissingType.ts]
+export interface I {
+    [key: string];
+}
+
+export class C {
+    readonly [key: number];
+}
+
+export type T = {
+    [key: string]
+    property: number;
+};
+
+export interface NumberIndex { [key: number]; }
+export interface SymbolIndex { [key: symbol]; }
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+export interface ExplicitAny { [key: string]: any; }
+export interface ExplicitUnknown { [key: string]: unknown; }
+
+declare const value: I;
+export const asNumber: number = value.anyKey;
+export const asString: string = value.anyKey;
+export const indexedValue = value.anyKey;
+
+declare const instance: C;
+declare const numbers: NumberIndex;
+declare const symbols: SymbolIndex;
+declare const templates: TemplateIndex;
+export const classValue = instance[0];
+export const numberValue = numbers[0];
+export const symbolValue = symbols[Symbol.iterator];
+export const templateValue = templates["prefix-foo"];
+instance[0] = 1;
+
+interface OptionalParameter {
+    [key?: string];
+}
+
+interface RestParameter {
+    [...key: string[]];
+}
+
+interface MultipleParameters {
+    [key: string, other: number];
+}
+
+interface InvalidKey {
+    [key: boolean];
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }
+
+
+//// [indexSignatureMissingType.js]
+export class C {
+}
+export const asNumber = value.anyKey;
+export const asString = value.anyKey;
+export const indexedValue = value.anyKey;
+export const classValue = instance[0];
+export const numberValue = numbers[0];
+export const symbolValue = symbols[Symbol.iterator];
+export const templateValue = templates["prefix-foo"];
+instance[0] = 1;
+
+
+//// [indexSignatureMissingType.d.ts]
+export interface I {
+    [key: string]: any;
+}
+export declare class C {
+    readonly [key: number]: any;
+}
+export type T = {
+    [key: string]: any;
+    property: number;
+};
+export interface NumberIndex {
+    [key: number]: any;
+}
+export interface SymbolIndex {
+    [key: symbol]: any;
+}
+export interface TemplateIndex {
+    [key: `prefix-${string}`]: any;
+}
+export interface ExplicitAny {
+    [key: string]: any;
+}
+export interface ExplicitUnknown {
+    [key: string]: unknown;
+}
+export declare const asNumber: number;
+export declare const asString: string;
+export declare const indexedValue: any;
+export declare const classValue: any;
+export declare const numberValue: any;
+export declare const symbolValue: any;
+export declare const templateValue: any;
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).symbols b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).symbols
new file mode 100644
index 0000000000000..8897bd254269d
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).symbols
@@ -0,0 +1,147 @@
+//// [tests/cases/compiler/indexSignatureMissingType.ts] ////
+
+=== indexSignatureMissingType.ts ===
+export interface I {
+>I : Symbol(I, Decl(indexSignatureMissingType.ts, 0, 0))
+
+    [key: string];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 1, 5))
+}
+
+export class C {
+>C : Symbol(C, Decl(indexSignatureMissingType.ts, 2, 1))
+
+    readonly [key: number];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 5, 14))
+}
+
+export type T = {
+>T : Symbol(T, Decl(indexSignatureMissingType.ts, 6, 1))
+
+    [key: string]
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 9, 5))
+
+    property: number;
+>property : Symbol(property, Decl(indexSignatureMissingType.ts, 9, 17))
+
+};
+
+export interface NumberIndex { [key: number]; }
+>NumberIndex : Symbol(NumberIndex, Decl(indexSignatureMissingType.ts, 11, 2))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 13, 32))
+
+export interface SymbolIndex { [key: symbol]; }
+>SymbolIndex : Symbol(SymbolIndex, Decl(indexSignatureMissingType.ts, 13, 47))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 14, 32))
+
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+>TemplateIndex : Symbol(TemplateIndex, Decl(indexSignatureMissingType.ts, 14, 47))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 15, 34))
+
+export interface ExplicitAny { [key: string]: any; }
+>ExplicitAny : Symbol(ExplicitAny, Decl(indexSignatureMissingType.ts, 15, 61))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 16, 32))
+
+export interface ExplicitUnknown { [key: string]: unknown; }
+>ExplicitUnknown : Symbol(ExplicitUnknown, Decl(indexSignatureMissingType.ts, 16, 52))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 17, 36))
+
+declare const value: I;
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>I : Symbol(I, Decl(indexSignatureMissingType.ts, 0, 0))
+
+export const asNumber: number = value.anyKey;
+>asNumber : Symbol(asNumber, Decl(indexSignatureMissingType.ts, 20, 12))
+>value.anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+
+export const asString: string = value.anyKey;
+>asString : Symbol(asString, Decl(indexSignatureMissingType.ts, 21, 12))
+>value.anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+
+export const indexedValue = value.anyKey;
+>indexedValue : Symbol(indexedValue, Decl(indexSignatureMissingType.ts, 22, 12))
+>value.anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+>value : Symbol(value, Decl(indexSignatureMissingType.ts, 19, 13))
+>anyKey : Symbol(I.__index, Decl(indexSignatureMissingType.ts, 0, 20))
+
+declare const instance: C;
+>instance : Symbol(instance, Decl(indexSignatureMissingType.ts, 24, 13))
+>C : Symbol(C, Decl(indexSignatureMissingType.ts, 2, 1))
+
+declare const numbers: NumberIndex;
+>numbers : Symbol(numbers, Decl(indexSignatureMissingType.ts, 25, 13))
+>NumberIndex : Symbol(NumberIndex, Decl(indexSignatureMissingType.ts, 11, 2))
+
+declare const symbols: SymbolIndex;
+>symbols : Symbol(symbols, Decl(indexSignatureMissingType.ts, 26, 13))
+>SymbolIndex : Symbol(SymbolIndex, Decl(indexSignatureMissingType.ts, 13, 47))
+
+declare const templates: TemplateIndex;
+>templates : Symbol(templates, Decl(indexSignatureMissingType.ts, 27, 13))
+>TemplateIndex : Symbol(TemplateIndex, Decl(indexSignatureMissingType.ts, 14, 47))
+
+export const classValue = instance[0];
+>classValue : Symbol(classValue, Decl(indexSignatureMissingType.ts, 28, 12))
+>instance : Symbol(instance, Decl(indexSignatureMissingType.ts, 24, 13))
+
+export const numberValue = numbers[0];
+>numberValue : Symbol(numberValue, Decl(indexSignatureMissingType.ts, 29, 12))
+>numbers : Symbol(numbers, Decl(indexSignatureMissingType.ts, 25, 13))
+
+export const symbolValue = symbols[Symbol.iterator];
+>symbolValue : Symbol(symbolValue, Decl(indexSignatureMissingType.ts, 30, 12))
+>symbols : Symbol(symbols, Decl(indexSignatureMissingType.ts, 26, 13))
+>Symbol.iterator : Symbol(SymbolConstructor.iterator, Decl(lib.es2015.iterable.d.ts, --, --))
+>Symbol : Symbol(Symbol, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.symbol.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2019.symbol.d.ts, --, --))
+>iterator : Symbol(SymbolConstructor.iterator, Decl(lib.es2015.iterable.d.ts, --, --))
+
+export const templateValue = templates["prefix-foo"];
+>templateValue : Symbol(templateValue, Decl(indexSignatureMissingType.ts, 31, 12))
+>templates : Symbol(templates, Decl(indexSignatureMissingType.ts, 27, 13))
+
+instance[0] = 1;
+>instance : Symbol(instance, Decl(indexSignatureMissingType.ts, 24, 13))
+
+interface OptionalParameter {
+>OptionalParameter : Symbol(OptionalParameter, Decl(indexSignatureMissingType.ts, 32, 16))
+
+    [key?: string];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 35, 5))
+}
+
+interface RestParameter {
+>RestParameter : Symbol(RestParameter, Decl(indexSignatureMissingType.ts, 36, 1))
+
+    [...key: string[]];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 39, 5))
+}
+
+interface MultipleParameters {
+>MultipleParameters : Symbol(MultipleParameters, Decl(indexSignatureMissingType.ts, 40, 1))
+
+    [key: string, other: number];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 43, 5))
+>other : Symbol(other, Decl(indexSignatureMissingType.ts, 43, 17))
+}
+
+interface InvalidKey {
+>InvalidKey : Symbol(InvalidKey, Decl(indexSignatureMissingType.ts, 44, 1))
+
+    [key: boolean];
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 47, 5))
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+>Ignored : Symbol(Ignored, Decl(indexSignatureMissingType.ts, 48, 1))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 51, 21))
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }
+>Expected : Symbol(Expected, Decl(indexSignatureMissingType.ts, 51, 36))
+>key : Symbol(key, Decl(indexSignatureMissingType.ts, 54, 22))
+
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).types b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).types
new file mode 100644
index 0000000000000..5d3d1bcdb256b
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMissingType(noimplicitany=true).types
@@ -0,0 +1,136 @@
+//// [tests/cases/compiler/indexSignatureMissingType.ts] ////
+
+=== indexSignatureMissingType.ts ===
+export interface I {
+    [key: string];
+>key : string
+}
+
+export class C {
+>C : C
+
+    readonly [key: number];
+>key : number
+}
+
+export type T = {
+>T : T
+
+    [key: string]
+>key : string
+
+    property: number;
+>property : number
+
+};
+
+export interface NumberIndex { [key: number]; }
+>key : number
+
+export interface SymbolIndex { [key: symbol]; }
+>key : symbol
+
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+>key : `prefix-${string}`
+
+export interface ExplicitAny { [key: string]: any; }
+>key : string
+
+export interface ExplicitUnknown { [key: string]: unknown; }
+>key : string
+
+declare const value: I;
+>value : I
+
+export const asNumber: number = value.anyKey;
+>asNumber : number
+>value.anyKey : any
+>value : I
+>anyKey : any
+
+export const asString: string = value.anyKey;
+>asString : string
+>value.anyKey : any
+>value : I
+>anyKey : any
+
+export const indexedValue = value.anyKey;
+>indexedValue : any
+>value.anyKey : any
+>value : I
+>anyKey : any
+
+declare const instance: C;
+>instance : C
+
+declare const numbers: NumberIndex;
+>numbers : NumberIndex
+
+declare const symbols: SymbolIndex;
+>symbols : SymbolIndex
+
+declare const templates: TemplateIndex;
+>templates : TemplateIndex
+
+export const classValue = instance[0];
+>classValue : any
+>instance[0] : any
+>instance : C
+>0 : 0
+
+export const numberValue = numbers[0];
+>numberValue : any
+>numbers[0] : any
+>numbers : NumberIndex
+>0 : 0
+
+export const symbolValue = symbols[Symbol.iterator];
+>symbolValue : any
+>symbols[Symbol.iterator] : any
+>symbols : SymbolIndex
+>Symbol.iterator : unique symbol
+>Symbol : SymbolConstructor
+>iterator : unique symbol
+
+export const templateValue = templates["prefix-foo"];
+>templateValue : any
+>templates["prefix-foo"] : any
+>templates : TemplateIndex
+>"prefix-foo" : "prefix-foo"
+
+instance[0] = 1;
+>instance[0] = 1 : 1
+>instance[0] : any
+>instance : C
+>0 : 0
+>1 : 1
+
+interface OptionalParameter {
+    [key?: string];
+>key : string | undefined
+}
+
+interface RestParameter {
+    [...key: string[]];
+>key : string[]
+}
+
+interface MultipleParameters {
+    [key: string, other: number];
+>key : string
+>other : number
+}
+
+interface InvalidKey {
+    [key: boolean];
+>key : boolean
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+>key : string
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }
+>key : boolean
+
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureMustHaveTypeAnnotation.errors.txt b/tsc/testdata/baselines/reference/compiler/indexSignatureMustHaveTypeAnnotation.errors.txt
index b9cc9dfed62af..37805c10625db 100644
--- a/tsc/testdata/baselines/reference/compiler/indexSignatureMustHaveTypeAnnotation.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureMustHaveTypeAnnotation.errors.txt
@@ -1,8 +1,8 @@
 indexSignatureMustHaveTypeAnnotation.ts(3,6): error TS2304: Cannot find name 'x'.
-indexSignatureMustHaveTypeAnnotation.ts(4,5): error TS1021: An index signature must have a type annotation.
+indexSignatureMustHaveTypeAnnotation.ts(4,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureMustHaveTypeAnnotation.ts(9,5): error TS2564: Property '[x]' has no initializer and is not definitely assigned in the constructor.
 indexSignatureMustHaveTypeAnnotation.ts(9,6): error TS2304: Cannot find name 'x'.
-indexSignatureMustHaveTypeAnnotation.ts(14,5): error TS1021: An index signature must have a type annotation.
+indexSignatureMustHaveTypeAnnotation.ts(14,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 
 
 ==== indexSignatureMustHaveTypeAnnotation.ts (5 errors) ====
@@ -13,7 +13,7 @@ indexSignatureMustHaveTypeAnnotation.ts(14,5): error TS1021: An index signature
 !!! error TS2304: Cannot find name 'x'.
         [x: string];
         ~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
     }
     
     class C {
@@ -29,5 +29,5 @@ indexSignatureMustHaveTypeAnnotation.ts(14,5): error TS1021: An index signature
     class C2 {
         [x: string]
         ~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
     }
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck.errors.txt b/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck.errors.txt
index 1633aa08f67c6..74e70e81eeb73 100644
--- a/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck.errors.txt
@@ -1,11 +1,15 @@
 indexSignatureTypeCheck.ts(8,5): error TS2322: Type 'null' is not assignable to type 'IPropertySet'.
+indexSignatureTypeCheck.ts(14,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck.ts(14,8): error TS1019: An index signature parameter cannot have a question mark.
+indexSignatureTypeCheck.ts(15,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck.ts(15,6): error TS1017: An index signature cannot have a rest parameter.
+indexSignatureTypeCheck.ts(16,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck.ts(16,6): error TS1096: An index signature must have exactly one parameter.
+indexSignatureTypeCheck.ts(17,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck.ts(17,6): error TS1096: An index signature must have exactly one parameter.
 
 
-==== indexSignatureTypeCheck.ts (5 errors) ====
+==== indexSignatureTypeCheck.ts (9 errors) ====
     interface IPropertySet {
     
         [index: string]: any;
@@ -22,15 +26,23 @@ indexSignatureTypeCheck.ts(17,6): error TS1096: An index signature must have exa
     
     interface indexErrors {
         [p2?: string];
+        ~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
            ~
 !!! error TS1019: An index signature parameter cannot have a question mark.
         [...p3: any[]];
+        ~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
          ~~~
 !!! error TS1017: An index signature cannot have a rest parameter.
         [p4: string, p5?: string];
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
          ~~
 !!! error TS1096: An index signature must have exactly one parameter.
         [p6: string, ...p7: any[]];
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
          ~~
 !!! error TS1096: An index signature must have exactly one parameter.
     }
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck2.errors.txt b/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck2.errors.txt
index 43c17fdc25b73..02fe24407c45a 100644
--- a/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck2.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureTypeCheck2.errors.txt
@@ -1,11 +1,15 @@
 indexSignatureTypeCheck2.ts(5,5): error TS2322: Type 'null' is not assignable to type 'IPropertySet'.
+indexSignatureTypeCheck2.ts(10,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck2.ts(10,8): error TS1019: An index signature parameter cannot have a question mark.
+indexSignatureTypeCheck2.ts(11,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck2.ts(11,6): error TS1017: An index signature cannot have a rest parameter.
+indexSignatureTypeCheck2.ts(12,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck2.ts(12,6): error TS1096: An index signature must have exactly one parameter.
+indexSignatureTypeCheck2.ts(13,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 indexSignatureTypeCheck2.ts(13,6): error TS1096: An index signature must have exactly one parameter.
 
 
-==== indexSignatureTypeCheck2.ts (5 errors) ====
+==== indexSignatureTypeCheck2.ts (9 errors) ====
     class IPropertySet {
         [index: string]: any
     }
@@ -18,15 +22,23 @@ indexSignatureTypeCheck2.ts(13,6): error TS1096: An index signature must have ex
     
     interface indexErrors {
         [p2?: string];
+        ~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
            ~
 !!! error TS1019: An index signature parameter cannot have a question mark.
         [...p3: any[]];
+        ~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
          ~~~
 !!! error TS1017: An index signature cannot have a rest parameter.
         [p4: string, p5?: string];
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
          ~~
 !!! error TS1096: An index signature must have exactly one parameter.
         [p6: string, ...p7: any[]];
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
          ~~
 !!! error TS1096: An index signature must have exactly one parameter.
     }
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexSignatureWithoutTypeAnnotation1.errors.txt b/tsc/testdata/baselines/reference/compiler/indexSignatureWithoutTypeAnnotation1.errors.txt
index 8e977b5799d36..83028fec60233 100644
--- a/tsc/testdata/baselines/reference/compiler/indexSignatureWithoutTypeAnnotation1.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexSignatureWithoutTypeAnnotation1.errors.txt
@@ -1,9 +1,9 @@
-indexSignatureWithoutTypeAnnotation1.ts(2,3): error TS1021: An index signature must have a type annotation.
+indexSignatureWithoutTypeAnnotation1.ts(2,3): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 
 
 ==== indexSignatureWithoutTypeAnnotation1.ts (1 errors) ====
     class C {
       [a: number];
       ~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
     }
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexTypeCheck.errors.txt b/tsc/testdata/baselines/reference/compiler/indexTypeCheck.errors.txt
index ccf0c99e3919d..27cbdd698b718 100644
--- a/tsc/testdata/baselines/reference/compiler/indexTypeCheck.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexTypeCheck.errors.txt
@@ -1,5 +1,3 @@
-indexTypeCheck.ts(2,2): error TS1021: An index signature must have a type annotation.
-indexTypeCheck.ts(3,2): error TS1021: An index signature must have a type annotation.
 indexTypeCheck.ts(17,2): error TS2413: 'number' index type 'number' is not assignable to 'string' index type 'string'.
 indexTypeCheck.ts(22,2): error TS2413: 'number' index type 'Orange' is not assignable to 'string' index type 'Yellow'.
 indexTypeCheck.ts(27,2): error TS2413: 'number' index type 'number' is not assignable to 'string' index type 'string'.
@@ -8,14 +6,10 @@ indexTypeCheck.ts(36,3): error TS1268: An index signature parameter type must be
 indexTypeCheck.ts(51,8): error TS2538: Type 'Blue' cannot be used as an index type.
 
 
-==== indexTypeCheck.ts (8 errors) ====
+==== indexTypeCheck.ts (6 errors) ====
     interface Red {
     	[n:number]; // ok
-    	~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
     	[s:string]; // ok
-    	~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
     }
     
     interface Blue {
diff --git a/tsc/testdata/baselines/reference/compiler/indexWithoutParamType.errors.txt b/tsc/testdata/baselines/reference/compiler/indexWithoutParamType.errors.txt
index 1483b3eb8b537..a39423b96148a 100644
--- a/tsc/testdata/baselines/reference/compiler/indexWithoutParamType.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexWithoutParamType.errors.txt
@@ -1,7 +1,10 @@
 indexWithoutParamType.ts(1,10): error TS1096: An index signature must have exactly one parameter.
+indexWithoutParamType.ts(1,10): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 
 
-==== indexWithoutParamType.ts (1 errors) ====
+==== indexWithoutParamType.ts (2 errors) ====
     var y: { []; } // Error
              ~~~
-!!! error TS1096: An index signature must have exactly one parameter.
\ No newline at end of file
+!!! error TS1096: An index signature must have exactly one parameter.
+             ~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/compiler/indexerConstraints2.errors.txt b/tsc/testdata/baselines/reference/compiler/indexerConstraints2.errors.txt
index ce0ebe20eb7ff..37c794986c319 100644
--- a/tsc/testdata/baselines/reference/compiler/indexerConstraints2.errors.txt
+++ b/tsc/testdata/baselines/reference/compiler/indexerConstraints2.errors.txt
@@ -8,7 +8,7 @@ indexerConstraints2.ts(52,6): error TS1337: An index signature parameter type ca
 indexerConstraints2.ts(58,6): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
 indexerConstraints2.ts(70,6): error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
 indexerConstraints2.ts(74,6): error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
-indexerConstraints2.ts(79,5): error TS1021: An index signature must have a type annotation.
+indexerConstraints2.ts(79,5): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 
 
 ==== indexerConstraints2.ts (11 errors) ====
@@ -112,6 +112,6 @@ indexerConstraints2.ts(79,5): error TS1021: An index signature must have a type
     interface T {
         [key: Key]
         ~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
     }
     
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/conformance/parserIndexSignature11.errors.txt b/tsc/testdata/baselines/reference/conformance/parserIndexSignature11.errors.txt
index 3e95b906714b6..07a90124afc21 100644
--- a/tsc/testdata/baselines/reference/conformance/parserIndexSignature11.errors.txt
+++ b/tsc/testdata/baselines/reference/conformance/parserIndexSignature11.errors.txt
@@ -1,16 +1,13 @@
 parserIndexSignature11.ts(2,10): error TS2304: Cannot find name 'p'.
-parserIndexSignature11.ts(3,9): error TS1021: An index signature must have a type annotation.
 parserIndexSignature11.ts(4,10): error TS1096: An index signature must have exactly one parameter.
 
 
-==== parserIndexSignature11.ts (3 errors) ====
+==== parserIndexSignature11.ts (2 errors) ====
     interface I {
             [p]; // Used to be indexer, now it is a computed property
              ~
 !!! error TS2304: Cannot find name 'p'.
             [p1: string];
-            ~~~~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
             [p2: string, p3: number];
              ~~
 !!! error TS1096: An index signature must have exactly one parameter.
diff --git a/tsc/testdata/baselines/reference/conformance/parserIndexSignature6.errors.txt b/tsc/testdata/baselines/reference/conformance/parserIndexSignature6.errors.txt
index c6f0788b6ffca..ddab49bda6add 100644
--- a/tsc/testdata/baselines/reference/conformance/parserIndexSignature6.errors.txt
+++ b/tsc/testdata/baselines/reference/conformance/parserIndexSignature6.errors.txt
@@ -1,9 +1,12 @@
+parserIndexSignature6.ts(2,3): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 parserIndexSignature6.ts(2,4): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
 
 
-==== parserIndexSignature6.ts (1 errors) ====
+==== parserIndexSignature6.ts (2 errors) ====
     interface I {
       [a:boolean]
+      ~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
        ~
 !!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
     }
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/conformance/parserIndexSignature7.errors.txt b/tsc/testdata/baselines/reference/conformance/parserIndexSignature7.errors.txt
index 6aac68380f312..c7ecf13094cf7 100644
--- a/tsc/testdata/baselines/reference/conformance/parserIndexSignature7.errors.txt
+++ b/tsc/testdata/baselines/reference/conformance/parserIndexSignature7.errors.txt
@@ -1,9 +1,9 @@
-parserIndexSignature7.ts(2,3): error TS1021: An index signature must have a type annotation.
+parserIndexSignature7.ts(2,3): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 
 
 ==== parserIndexSignature7.ts (1 errors) ====
     interface I {
       [a:string]
       ~~~~~~~~~~
-!!! error TS1021: An index signature must have a type annotation.
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
     }
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/conformance/parserIndexSignature8.errors.txt b/tsc/testdata/baselines/reference/conformance/parserIndexSignature8.errors.txt
index 44bd4aeeb92c6..b132de1e7be5a 100644
--- a/tsc/testdata/baselines/reference/conformance/parserIndexSignature8.errors.txt
+++ b/tsc/testdata/baselines/reference/conformance/parserIndexSignature8.errors.txt
@@ -1,12 +1,18 @@
+parserIndexSignature8.ts(1,12): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 parserIndexSignature8.ts(1,13): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
+parserIndexSignature8.ts(2,13): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 parserIndexSignature8.ts(2,14): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
 
 
-==== parserIndexSignature8.ts (2 errors) ====
+==== parserIndexSignature8.ts (4 errors) ====
     var foo: { [index: any]; }; // expect an error here
+               ~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
                 ~~~~~
 !!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
     var foo2: { [index: RegExp]; }; // expect an error here
+                ~~~~~~~~~~~~~~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
                  ~~~~~
 !!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
     
\ No newline at end of file
diff --git a/tsc/testdata/baselines/reference/conformance/parserObjectType6.errors.txt b/tsc/testdata/baselines/reference/conformance/parserObjectType6.errors.txt
index aa8fe99484a1b..e2eaeb66b104c 100644
--- a/tsc/testdata/baselines/reference/conformance/parserObjectType6.errors.txt
+++ b/tsc/testdata/baselines/reference/conformance/parserObjectType6.errors.txt
@@ -1,8 +1,9 @@
 parserObjectType6.ts(2,7): error TS2304: Cannot find name 'B'.
 parserObjectType6.ts(3,4): error TS1096: An index signature must have exactly one parameter.
+parserObjectType6.ts(3,4): error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
 
 
-==== parserObjectType6.ts (2 errors) ====
+==== parserObjectType6.ts (3 errors) ====
     var v: {
        a: B
           ~
@@ -10,4 +11,6 @@ parserObjectType6.ts(3,4): error TS1096: An index signature must have exactly on
        [];
        ~~~
 !!! error TS1096: An index signature must have exactly one parameter.
+       ~~~
+!!! error TS7062: Index signature, which lacks type annotation, implicitly has an 'any' type.
     };
\ No newline at end of file
diff --git a/tsc/testdata/tests/cases/compiler/indexSignatureMissingType.ts b/tsc/testdata/tests/cases/compiler/indexSignatureMissingType.ts
new file mode 100644
index 0000000000000..e5f82c0756b04
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/indexSignatureMissingType.ts
@@ -0,0 +1,59 @@
+// @declaration: true
+// @noImplicitAny: false, true
+// @target: esnext
+
+export interface I {
+    [key: string];
+}
+
+export class C {
+    readonly [key: number];
+}
+
+export type T = {
+    [key: string]
+    property: number;
+};
+
+export interface NumberIndex { [key: number]; }
+export interface SymbolIndex { [key: symbol]; }
+export interface TemplateIndex { [key: `prefix-${string}`]; }
+export interface ExplicitAny { [key: string]: any; }
+export interface ExplicitUnknown { [key: string]: unknown; }
+
+declare const value: I;
+export const asNumber: number = value.anyKey;
+export const asString: string = value.anyKey;
+export const indexedValue = value.anyKey;
+
+declare const instance: C;
+declare const numbers: NumberIndex;
+declare const symbols: SymbolIndex;
+declare const templates: TemplateIndex;
+export const classValue = instance[0];
+export const numberValue = numbers[0];
+export const symbolValue = symbols[Symbol.iterator];
+export const templateValue = templates["prefix-foo"];
+instance[0] = 1;
+
+interface OptionalParameter {
+    [key?: string];
+}
+
+interface RestParameter {
+    [...key: string[]];
+}
+
+interface MultipleParameters {
+    [key: string, other: number];
+}
+
+interface InvalidKey {
+    [key: boolean];
+}
+
+// @ts-ignore
+interface Ignored { [key: string]; }
+
+// @ts-expect-error
+interface Expected { [key: boolean]; }