[clang][Interp] Test non-active union access

Author: tbaederr

clang/lib/AST/Interp/Interp.cpp

@@ -457,10 +457,10 @@ bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
     return false;
   if (!CheckRange(S, OpPC, Ptr, AK_Read))
     return false;
-  if (!CheckInitialized(S, OpPC, Ptr, AK_Read))
-    return false;
   if (!CheckActive(S, OpPC, Ptr, AK_Read))
     return false;
+  if (!CheckInitialized(S, OpPC, Ptr, AK_Read))
+    return false;
   if (!CheckTemporary(S, OpPC, Ptr, AK_Read))
     return false;
   if (!CheckMutable(S, OpPC, Ptr))

clang/test/AST/Interp/unions.cpp

@@ -1,14 +1,12 @@
 // RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify=expected,both %s
 // RUN: %clang_cc1 -verify=ref,both %s
 
-// both-no-diagnostics
-
 union U {
   int a;
   int b;
 };
 
 constexpr U a = {12};
 static_assert(a.a == 12, "");
-
-
+static_assert(a.b == 0, ""); // both-error {{not an integral constant expression}} \
+                             // both-note {{read of member 'b' of union with active member 'a'}}