[clang][Interp] Only diagnose out of bounds enum values in C++

Author: tbaederr

clang/lib/AST/Interp/Compiler.cpp

@@ -468,7 +468,7 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
 
     // Possibly diagnose casts to enum types if the target type does not
     // have a fixed size.
-    if (CE->getType()->isEnumeralType()) {
+    if (Ctx.getLangOpts().CPlusPlus && CE->getType()->isEnumeralType()) {
       if (const auto *ET = CE->getType().getCanonicalType()->getAs<EnumType>();
           ET && !ET->getDecl()->isFixed()) {
         if (!this->emitCheckEnumValue(*FromT, ET->getDecl(), CE))

clang/test/Sema/switch.c

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wswitch-enum -Wcovered-switch-default -triple x86_64-linux-gnu %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wswitch-enum -Wcovered-switch-default -triple x86_64-linux-gnu %s -fexperimental-new-constant-interpreter
 void f (int z) { 
   while (z) { 
     default: z--;            // expected-error {{statement not in switch}}