re PR c++/89481 (constexpr function allows writing one active union member and reading another)

	PR c++/89481
	* constexpr.c (cxx_eval_store_expression): When changing active union
	member, set no_zero_init.

	* g++.dg/cpp1y/constexpr-89481.C: New test.

From-SVN: r269213

gcc/cp/ChangeLog

+2019-02-26  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/89481
+	* constexpr.c (cxx_eval_store_expression): When changing active union
+	member, set no_zero_init.
+
 2019-02-23  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/88294 - ICE with non-constant noexcept-specifier.

gcc/cp/constexpr.c

@@ -3860,6 +3860,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 		}
 	      /* Changing active member.  */
 	      vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
+	      no_zero_init = true;
 	    }
 
 	  for (idx = 0;

gcc/testsuite/ChangeLog

+2019-02-26  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/89481
+	* g++.dg/cpp1y/constexpr-89481.C: New test.
+
 2019-02-26  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/89505

gcc/testsuite/g++.dg/cpp1y/constexpr-89481.C

+// PR c++/89481
+// { dg-do compile { target c++14 } }
+
+constexpr int
+foo ()
+{
+  union U { long long a; int b[2]; } u { 5LL };
+  u.b[1] = 4;		// { dg-error "change of the active member of a union from" "" { target c++17_down } }
+  return u.b[0];
+}
+
+constexpr int
+bar ()
+{
+  union U { long long a; int b[2]; } u { 5LL };
+  u.b[1] = 4;		// { dg-error "change of the active member of a union from" "" { target c++17_down } }
+  return u.b[1];
+}
+
+static_assert (foo () == 0, "");	// { dg-error "non-constant condition for static assertion" }
+					// { dg-message "in 'constexpr' expansion of" "" { target *-*-* } .-1 }
+					// { dg-error "accessing uninitialized array element" "" { target c++2a } .-2 }
+static_assert (bar () == 4, "");	// { dg-error "non-constant condition for static assertion" "" { target c++17_down } }
+					// { dg-message "in 'constexpr' expansion of" "" { target c++17_down } .-1 }