c++: constexpr union member access folding [PR114709]

The object/offset canonicalization performed in cxx_fold_indirect_ref
is undesirable for union member accesses because it loses information
about the member being accessed which we may later need to diagnose an
inactive-member access.  So this patch restricts the canonicalization
accordingly.

	PR c++/114709

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_fold_indirect_ref): Restrict object/offset
	canonicalization to RECORD_TYPE member accesses.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-union8.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/constexpr.cc

@@ -5799,6 +5799,9 @@ cxx_fold_indirect_ref (const constexpr_ctx *ctx, location_t loc, tree type,
      more folding opportunities.  */
   auto canonicalize_obj_off = [] (tree& obj, tree& off) {
     while (TREE_CODE (obj) == COMPONENT_REF
+	   /* We need to preserve union member accesses so that we can
+	      later properly diagnose accessing the wrong member.  */
+	   && TREE_CODE (TREE_TYPE (TREE_OPERAND (obj, 0))) == RECORD_TYPE
 	   && (tree_int_cst_sign_bit (off) || integer_zerop (off)))
       {
 	tree field = TREE_OPERAND (obj, 1);

gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C

@@ -0,0 +1,8 @@
+// PR c++/114709
+// { dg-do compile { target c++11 } }
+
+struct T1 { int a, b; };
+struct T2 { int c; double d; };
+union U { T1 t1; T2 t2; };
+
+constexpr int v = U{{1,2}}.t2.*&T2::c; // { dg-error "accessing 'U::t2'" }