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Fix #711 (#if true evaluates to false in C++ and C23) #712

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Merged
danmar merged 1 commit into from
Nov 5, 2026
Merged

Fix #711 (#if true evaluates to false in C++ and C23) #712

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12 changes: 10 additions & 2 deletions simplecpp.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2824,10 +2824,18 @@ static void simplifyHasInclude(simplecpp::TokenList &expr, const simplecpp::DUI
/** Evaluate name
* @throws std::runtime_error thrown on undefined function-like macro
*/
static void simplifyName(simplecpp::TokenList &expr)
static void simplifyName(simplecpp::TokenList &expr, const simplecpp::DUI &dui)
{
// In C++ and C23 the keywords true/false are 1/0 in #if expressions
const simplecpp::cstd_t cstd = simplecpp::getCStd(dui.std);
const bool hasTrueFalse = (cstd == simplecpp::CUnknown || cstd >= simplecpp::C23);
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for (simplecpp::Token *tok = expr.front(); tok; tok = tok->next) {
if (tok->name) {
if (hasTrueFalse && (tok->str() == "true" || tok->str() == "false")) {
tok->setstr(tok->str() == "true" ? "1" : "0");
continue;
}
static const std::set<std::string> altop{"and","or","bitand","bitor","compl","not","not_eq","xor"};
if (altop.find(tok->str()) != altop.end()) {
bool alt;
Expand Down Expand Up @@ -3101,7 +3109,7 @@ static long long evaluate(simplecpp::TokenList &expr, const simplecpp::DUI &dui,
simplifyComments(expr);
simplifySizeof(expr, sizeOfType);
simplifyHasInclude(expr, dui);
simplifyName(expr);
simplifyName(expr, dui);
simplifyNumbers(expr);
expr.constFold();
// TODO: handle invalid expressions
Expand Down
19 changes: 19 additions & 0 deletions test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2609,6 +2609,24 @@ static void ifdiv0()
ASSERT_EQUALS("", preprocess(code));
}

static void ifTrueFalse()
{
const char code[] = "#if true && !false\n"
"1\n"
"#else\n"
"2\n"
"#endif\n";
simplecpp::DUI dui;
ASSERT_EQUALS("\n1", preprocess(code, dui, nullptr));
dui.std = "c++11";
ASSERT_EQUALS("\n1", preprocess(code, dui, nullptr));
dui.std = "c23";
ASSERT_EQUALS("\n1", preprocess(code, dui, nullptr));
// before C23, true and false are ordinary identifiers that evaluate to 0
dui.std = "c17";
ASSERT_EQUALS("\n\n\n2", preprocess(code, dui, nullptr));
}

static void ifalt() // using "and", "or", etc
{
const char *code;
Expand Down Expand Up @@ -4794,6 +4812,7 @@ static void runTests(int argc, char **argv, Input input)
TEST_CASE(ifif);
TEST_CASE(ifoverflow);
TEST_CASE(ifdiv0);
TEST_CASE(ifTrueFalse);
TEST_CASE(ifalt); // using "and", "or", etc
TEST_CASE(ifexpr);
TEST_CASE(ifUndefFuncStyleMacro);
Expand Down
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