From 321020d2248e8ea0c42369b8a2e02de98c150b83 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Marjam=C3=A4ki?= <daniel.marjamaki@cppcheck.com>
Date: Sat, 7 Nov 2026 12:19:56 +0100
Subject: [PATCH] Fix #727 (#if: two unary operators in a row silently evaluate
 to false)

Unary operators were folded from left to right in one pass, so in "!-0" the
! was skipped because its operand was not folded yet. Fold them from right to
left. Negating a negative value no longer gives an invalid number such as "--1".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 simplecpp.cpp | 16 +++++++++++++---
 test.cpp      | 30 ++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/simplecpp.cpp b/simplecpp.cpp
index 60f879cc..c9d762fa 100644
--- a/simplecpp.cpp
+++ b/simplecpp.cpp
@@ -1291,9 +1291,16 @@ void simplecpp::TokenList::combineOperators()
 
 static const std::string COMPL("compl");
 static const std::string NOT("not");
-void simplecpp::TokenList::constFoldUnaryNotPosNeg(simplecpp::Token *tok)
+void simplecpp::TokenList::constFoldUnaryNotPosNeg(simplecpp::Token *tok1)
 {
-    for (; tok && tok->op != ')'; tok = tok->next) {
+    if (!tok1 || tok1->op == ')')
+        return;
+    Token *last = tok1;
+    while (last->next && last->next->op != ')')
+        last = last->next;
+
+    // unary operators are right-associative, "!-0" means "!(-0)", so they are folded from right to left
+    for (Token *tok = last; tok; tok = (tok == tok1) ? nullptr : tok->previous) {
         // "not" might be !
         if (isAlternativeUnaryOp(tok, NOT))
             tok->op = '!';
@@ -1328,7 +1335,10 @@ void simplecpp::TokenList::constFoldUnaryNotPosNeg(simplecpp::Token *tok)
                 deleteToken(tok->next);
                 break;
             case '-':
-                tok->setstr(tok->op + tok->next->str());
+                if (tok->next->str()[0] == '-')
+                    tok->setstr(tok->next->str().substr(1));
+                else
+                    tok->setstr(tok->op + tok->next->str());
                 deleteToken(tok->next);
                 break;
             }
diff --git a/test.cpp b/test.cpp
index 1296c790..00a4a136 100644
--- a/test.cpp
+++ b/test.cpp
@@ -2913,6 +2913,35 @@ static void ifZeroCondition()
     ASSERT_EQUALS("file0,1,syntax_error,failed to evaluate #if condition, division/modulo by zero\n", toString(outputList));
 }
 
+static void ifUnaryOperators()
+{
+    // unary operators in a row
+    const char * const exprs[] = {
+        "!!1",
+        "!-0",
+        "!+0",
+        "!~-1",
+        "- -1",
+        "-!0",
+        "-~0",
+        "~!0",
+        "+-1 == -1",
+        "1 && !-0",
+        "!-X",
+        "-(-1) == 1",
+        "- - -1 == -1",
+        "1 - -1 == 2"
+    };
+    for (const char * const expr : exprs) {
+        const std::string code = std::string("#if ") + expr + "\n"
+                                 "1\n"
+                                 "#else\n"
+                                 "2\n"
+                                 "#endif\n";
+        ASSERT_EQUALS("\n1", preprocess(code.c_str()));
+    }
+}
+
 static void ifTrueFalse()
 {
     const char code[] = "#if true && !false\n"
@@ -5150,6 +5179,7 @@ static void runTests(int argc, char **argv, Input input)
     TEST_CASE(ifdiv0);
     TEST_CASE(ifShortCircuit);
     TEST_CASE(ifZeroCondition);
+    TEST_CASE(ifUnaryOperators);
     TEST_CASE(ifTrueFalse);
     TEST_CASE(ifalt); // using "and", "or", etc
     TEST_CASE(ifexpr);