Evaluate union cast subexpressions when the cast value is unused

Fixes PR23597.

llvm-svn: 237839

clang/lib/CodeGen/CGExprAgg.cpp

@@ -584,7 +584,12 @@ void AggExprEmitter::VisitCastExpr(CastExpr *E) {
   }
       
   case CK_ToUnion: {
-    if (Dest.isIgnored()) break;
+    // Evaluate even if the destination is ignored.
+    if (Dest.isIgnored()) {
+      CGF.EmitAnyExpr(E->getSubExpr(), AggValueSlot::ignored(),
+                      /*ignoreResult=*/true);
+      break;
+    }
 
     // GCC union extension
     QualType Ty = E->getSubExpr()->getType();

clang/test/CodeGen/exprs.c

@@ -184,3 +184,14 @@ void f17() {
   extfunc(x);
 // CHECK: add nsw i128 %{{.}}, -1
 }
+
+// PR23597: We should evaluate union cast operands even if the cast is unused.
+typedef union u {
+    int i;
+} strct;
+int returns_int(void);
+void f18() {
+  (strct)returns_int();
+}
+// CHECK-LABEL: define void @f18()
+// CHECK: call i32 @returns_int()