The controlling expression for _Generic is unevaluated

[aaron.ballman]

Summary

In r252104, I fixed a _Generic bug so that the controlling expression has its type decayed and qualifiers stripped. However, this caused a diagnostic regression because the controlling expression is not evaluated, and my fix triggered diagnostics (like null pointer dereferences) that it should not have. This patch fixes the regression by putting the evaluation of the controlling expression into an unevaluated context before decaying and stripping qualifiers.

Assuming this patch is acceptable, I think it should go into 3.8 as well.

[rsmith]

LGTM for trunk and 3.8.

[aaron.ballman]

Thanks! I've commit in r261669.

lib/Sema/SemaExpr.cpp

 
   // Decay and strip qualifiers for the controlling expression type, and handle
   // placeholder type replacement. See committee discussion from WG14 DR423.
-  ExprResult R = DefaultFunctionArrayLvalueConversion(ControllingExpr);
-  if (R.isInvalid())
-    return ExprError();
-  ControllingExpr = R.get();
+  {
+    EnterExpressionEvaluationContext Unevaluated(*this, Sema::Unevaluated);
+    ExprResult R = DefaultFunctionArrayLvalueConversion(ControllingExpr);
+    if (R.isInvalid())
+      return ExprError();
+    ControllingExpr = R.get();
+  }
 
   // The controlling expression is an unevaluated operand, so side effects are
   // likely unintended.

test/Sema/generic-selection.c

 
   const int i = 12;
   int a9[_Generic(i, int: 1, default: 2) == 1 ? 1 : -1];
+
+  // This is expected to not trigger any diagnostics because the controlling
+  // expression is not evaluated.
+  (void)_Generic(*(int *)0, int: 1);
 }