[clang] [clang] Reject if constexpr in C (PR #112685)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Mariya Podchishchaeva (Fznamznon)

<details>
<summary>Changes</summary>

Fixes https://github.com/llvm/llvm-project/issues/112587

---
Full diff: https://github.com/llvm/llvm-project/pull/112685.diff


2 Files Affected:

- (modified) clang/lib/Parse/ParseStmt.cpp (+7-4) 
- (modified) clang/test/Sema/constexpr.c (+7) 


``````````diff
diff --git a/clang/lib/Parse/ParseStmt.cpp b/clang/lib/Parse/ParseStmt.cpp
index 6480e88316a7d5..60d647da48f053 100644
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@@ -1518,10 +1518,13 @@ StmtResult Parser::ParseIfStatement(SourceLocation *TrailingElseLoc) {
   SourceLocation ConstevalLoc;
 
   if (Tok.is(tok::kw_constexpr)) {
-    Diag(Tok, getLangOpts().CPlusPlus17 ? diag::warn_cxx14_compat_constexpr_if
-                                        : diag::ext_constexpr_if);
-    IsConstexpr = true;
-    ConsumeToken();
+    // C23 supports constexpr keyword, but only for object definitions.
+    if (getLangOpts().CPlusPlus) {
+      Diag(Tok, getLangOpts().CPlusPlus17 ? diag::warn_cxx14_compat_constexpr_if
+                                          : diag::ext_constexpr_if);
+      IsConstexpr = true;
+      ConsumeToken();
+    }
   } else {
     if (Tok.is(tok::exclaim)) {
       NotLocation = ConsumeToken();
diff --git a/clang/test/Sema/constexpr.c b/clang/test/Sema/constexpr.c
index 0cf9491c4a42bf..eaa000b3b97758 100644
--- a/clang/test/Sema/constexpr.c
+++ b/clang/test/Sema/constexpr.c
@@ -367,3 +367,10 @@ struct S10 {
 constexpr struct S10 c = { 255 };
 // FIXME-expected-error at -1 {{constexpr initializer evaluates to 255 which is not exactly representable in 'long long' bit-field with width 8}}
 // See: GH#101299
+
+void constexprif() {
+  if constexpr (300) {} //expected-error {{expected '(' after 'if'}}
+}
+void constevalif() {
+  if consteval (300) {} //expected-error {{expected '(' after 'if'}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/112685