Add another test case from review feedback

Author: AaronBallman

clang/test/AST/ByteCode/literals.cpp

@@ -886,6 +886,19 @@ namespace CompoundLiterals {
   }
   constexpr int g = f2(); // Should evaluate to 1?
   static_assert(g == 1, "");
+
+  // This example should be rejected because the lifetime of the compound
+  // literal assigned into x is that of the full expression, which is the
+  // parenthesized assignment operator. So the return statement is using a
+  // dangling pointer. FIXME: the note saying it's a read of a dereferenced
+  // null pointer suggests we're doing something odd during constant expression
+  // evaluation: I think it's still taking 'x' as being null from the call to
+  // f3() rather than tracking the assignment happening in the VLA.
+  constexpr int f3(int *x, int (*y)[*(x=(int[]){1,2,3})]) { // both-warning {{object backing the pointer x will be destroyed at the end of the full-expression}}
+    return x[0]; // both-note {{read of dereferenced null pointer is not allowed in a constant expression}}
+  }
+  constexpr int h = f3(0,0); // both-error {{constexpr variable 'h' must be initialized by a constant expression}} \
+                                both-note {{in call to 'f3(nullptr, nullptr)'}}
 }
 
 namespace TypeTraits {