[clang][Interp] Handle discarded PointerToIntegral casts

Resolve an old TODO comment.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -167,7 +167,9 @@ bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
     return this->emitNull(classifyPrim(CE->getType()), CE);
 
   case CK_PointerToIntegral: {
-    // TODO: Discard handling.
+    if (DiscardResult)
+      return this->discard(SubExpr);
+
     if (!this->visit(SubExpr))
       return false;
 

clang/test/AST/Interp/c.c

@@ -24,6 +24,7 @@ _Static_assert(!!1, "");
 int a = (1 == 1 ? 5 : 3);
 _Static_assert(a == 5, ""); // all-error {{not an integral constant expression}}
 
+const int DiscardedPtrToIntCast = ((intptr_t)((void*)0), 0); // all-warning {{left operand of comma operator has no effect}}
 
 const int b = 3;
 _Static_assert(b == 3, ""); // pedantic-ref-warning {{not an integer constant expression}} \