[clang][bytecode] Reject assignments in C (llvm#136126)

Similar to what the current interpreter does.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -863,8 +863,12 @@ bool Compiler<Emitter>::VisitBinaryOperator(const BinaryOperator *BO) {
       return this->VisitPointerArithBinOp(BO);
   }
 
-  // Assignmentes require us to evalute the RHS first.
+  // Assignments require us to evalute the RHS first.
   if (BO->getOpcode() == BO_Assign) {
+    // We don't support assignments in C.
+    if (!Ctx.getLangOpts().CPlusPlus)
+      return this->emitInvalid(BO);
+
     if (!visit(RHS) || !visit(LHS))
       return false;
     if (!this->emitFlip(*LT, *RT, BO))

clang/test/AST/ByteCode/c2y.c

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-linux %s -std=c2y -verify=expected,both -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -triple x86_64-linux %s -std=c2y -verify=ref,both
+
+// both-no-diagnostics
+
+struct S {
+  int x;
+  char c;
+  float f;
+};
+
+#define DECL_BUFFER(Ty, Name) alignas(Ty) unsigned char Name[sizeof(Ty)]
+
+struct T {
+  DECL_BUFFER(struct S, buffer);
+};
+
+int quorble() {
+  DECL_BUFFER(struct T, buffer);
+  ((struct S *)((struct T *)buffer)->buffer)->x = 12;
+  const struct S *s_ptr = (struct S *)((struct T *)buffer)->buffer;
+  return s_ptr->x;
+}