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

Fixes llvm#112587

Author: Fznamznon

clang/lib/Parse/ParseStmt.cpp

@@ -1508,10 +1508,13 @@ 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

@@ -357,3 +357,10 @@ void infsNaNs() {
   constexpr double db5 = LD_SNAN; // expected-error {{constexpr initializer evaluates to nan which is not exactly representable in type 'const double'}}
   constexpr double db6 = INF;
 }
+
+void constexprif() {
+  if constexpr (300) {} //expected-error {{expected '(' after 'if'}}
+}
+void constevalif() {
+  if consteval (300) {} //expected-error {{expected '(' after 'if'}}
+}