From 7fad4481ddb6a6000c28c6d60f896e564e8bff6c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Marjam=C3=A4ki?= <daniel.marjamaki@cppcheck.com>
Date: Sat, 7 Nov 2026 13:40:41 +0100
Subject: [PATCH] Fix #731 (#if: an operator with a missing operand silently
 evaluates to false)

Report "invalid expression" when the #if expression can not be folded to a
single number. This also covers a missing operator ("1 2") and empty
parentheses ("()").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 simplecpp.cpp |  5 +++--
 test.cpp      | 46 +++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/simplecpp.cpp b/simplecpp.cpp
index c9d762fa..48a7ce9e 100644
--- a/simplecpp.cpp
+++ b/simplecpp.cpp
@@ -3283,8 +3283,9 @@ static long long evaluate(simplecpp::TokenList &expr, const simplecpp::DUI &dui,
     simplifyName(expr, dui);
     simplifyNumbers(expr);
     expr.constFold();
-    // TODO: handle invalid expressions
-    return expr.cfront() && expr.cfront() == expr.cback() && expr.cfront()->number ? stringToLL(expr.cfront()->str()) : 0LL;
+    if (!expr.cfront() || expr.cfront() != expr.cback() || !expr.cfront()->number)
+        throw std::runtime_error("invalid expression");
+    return stringToLL(expr.cfront()->str());
 }
 
 static const simplecpp::Token *gotoNextLine(const simplecpp::Token *tok)
diff --git a/test.cpp b/test.cpp
index 00a4a136..866af490 100644
--- a/test.cpp
+++ b/test.cpp
@@ -1842,7 +1842,8 @@ static void hashEscapeTrailingBackslash()
                             "#endif\n";
         simplecpp::OutputList outputList;
         preprocess(code, &outputList);
-        ASSERT_EQUALS(std::string("file0,2,portability_stringify,") + msg, toString(outputList));
+        ASSERT_EQUALS(std::string("file0,2,portability_stringify,") + msg +
+                      "file0,2,syntax_error,failed to evaluate #if condition, invalid expression\n", toString(outputList));
     }
     {
         // even number of backslashes is fine
@@ -2942,6 +2943,48 @@ static void ifUnaryOperators()
     }
 }
 
+static void ifInvalidExpression()
+{
+    const char * const exprs[] = {
+        // missing operand
+        "1 +",
+        "(1 +)",
+        "(1 +) + 1",
+        "* 1",
+        "(* 1)",
+        "1 + * 2",
+        "1 <<",
+        "(1 ==)",
+        "1 &",
+        "(1 &&)",
+        "|| 1",
+        "!",
+        "(!)",
+        "-",
+        "~",
+        "(? 1 : 2)",
+        "(1 ? 2 :)",
+        "(1 ? 2 3)",
+        "(1 ? : 2)",
+        // missing operator
+        "1 2",
+        "(1 2)",
+        // empty parentheses
+        "()",
+        "1 + ()",
+        // syntax errors are reported also in operands that are not evaluated
+        "0 && (1 +)"
+    };
+    for (const char * const expr : exprs) {
+        const std::string code = std::string("#if ") + expr + "\n"
+                                 "1\n"
+                                 "#endif\n";
+        simplecpp::OutputList outputList;
+        ASSERT_EQUALS("", preprocess(code.c_str(), &outputList));
+        ASSERT_EQUALS("file0,1,syntax_error,failed to evaluate #if condition, invalid expression\n", toString(outputList));
+    }
+}
+
 static void ifTrueFalse()
 {
     const char code[] = "#if true && !false\n"
@@ -5180,6 +5223,7 @@ static void runTests(int argc, char **argv, Input input)
     TEST_CASE(ifShortCircuit);
     TEST_CASE(ifZeroCondition);
     TEST_CASE(ifUnaryOperators);
+    TEST_CASE(ifInvalidExpression);
     TEST_CASE(ifTrueFalse);
     TEST_CASE(ifalt); // using "and", "or", etc
     TEST_CASE(ifexpr);