[clang][Interp] Ignore LValueToRValue casts before doing the load

If the SubExpr results in an invalid pointer, we will otherwise
reject the constant expression.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -78,6 +78,9 @@ bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
   switch (CE->getCastKind()) {
 
   case CK_LValueToRValue: {
+    if (DiscardResult)
+      return this->discard(SubExpr);
+
     return dereference(
         SubExpr, DerefKind::Read,
         [](PrimType) {
@@ -86,9 +89,7 @@ bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
         },
         [this, CE](PrimType T) {
           // Pointer on stack - dereference it.
-          if (!this->emitLoadPop(T, CE))
-            return false;
-          return DiscardResult ? this->emitPop(T, CE) : true;
+          return this->emitLoadPop(T, CE);
         });
   }
 

clang/test/AST/Interp/c.c

@@ -98,3 +98,15 @@ void f (int z) {
 
 int expr;
 int chooseexpr[__builtin_choose_expr(1, 1, expr)];
+
+int somefunc(int i) {
+  return (i, 65537) * 65537; // expected-warning {{left operand of comma operator has no effect}} \
+                             // expected-warning {{overflow in expression; result is 131073}} \
+                             // pedantic-expected-warning {{left operand of comma operator has no effect}} \
+                             // pedantic-expected-warning {{overflow in expression; result is 131073}} \
+                             // ref-warning {{left operand of comma operator has no effect}} \
+                             // ref-warning {{overflow in expression; result is 131073}} \
+                             // pedantic-ref-warning {{left operand of comma operator has no effect}} \
+                             // pedantic-ref-warning {{overflow in expression; result is 131073}}
+
+}