Fix ptrmem comparison for unions.

	* constexpr.c (cxx_eval_binary_expression): Special case comparison
	of pointers to members of the same union.

From-SVN: r261454

gcc/cp/ChangeLog

+2018-06-05  Jason Merrill  <jason@redhat.com>
+
+	* constexpr.c (cxx_eval_binary_expression): Special case comparison
+	of pointers to members of the same union.
+
 2018-06-11  Jason Merrill  <jason@redhat.com>
 
 	PR c++/86094 - wrong code with defaulted move ctor.

gcc/cp/constexpr.c

@@ -2051,8 +2051,22 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
 
       if (TREE_CODE (lhs) == PTRMEM_CST
 	  && TREE_CODE (rhs) == PTRMEM_CST)
-	r = constant_boolean_node (cp_tree_equal (lhs, rhs) == is_code_eq,
-				   type);
+	{
+	  tree lmem = PTRMEM_CST_MEMBER (lhs);
+	  tree rmem = PTRMEM_CST_MEMBER (rhs);
+	  bool eq;
+	  if (TREE_CODE (lmem) == TREE_CODE (rmem)
+	      && TREE_CODE (lmem) == FIELD_DECL
+	      && TREE_CODE (DECL_CONTEXT (lmem)) == UNION_TYPE
+	      && same_type_p (DECL_CONTEXT (lmem),
+			      DECL_CONTEXT (rmem)))
+	    /* If both refer to (possibly different) members of the same union
+	       (12.3), they compare equal. */
+	    eq = true;
+	  else
+	    eq = cp_tree_equal (lhs, rhs);
+	  r = constant_boolean_node (eq == is_code_eq, type);
+	}
       else if ((TREE_CODE (lhs) == PTRMEM_CST
 		|| TREE_CODE (rhs) == PTRMEM_CST)
 	       && (null_member_pointer_value_p (lhs)

gcc/testsuite/g++.dg/expr/ptrmem10.C

+/* [expr.eq] If both refer to (possibly different) members of the same union
+   (12.3), they compare equal. */
+// { dg-do run { target c++11 } }
+// { dg-additional-options -O }
+
+union U
+{
+  int i;
+  int j;
+};
+
+#define SA(X) static_assert ((X),#X)
+SA (&U::i == &U::j);
+SA (!(&U::i != &U::j));
+
+#define assert(X) do { if (!(X)) __builtin_abort(); } while(0)
+
+void f (int U::*p, int U::*q)
+{
+  assert (p==q);
+  assert (!(p!=q));
+}
+
+int main()
+{
+  assert (&U::i == &U::j);
+  assert (!(&U::i != &U::j));
+}