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

Fixes #112587

Author: Fznamznon

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();

clang/test/Sema/constexpr.c

@@ -367,3 +367,10 @@ struct S10 {
 constexpr struct S10 c = { 255 };
 // FIXME-expected-error@-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'}}
+}