PR c++/89336 - multiple stores in constexpr stmt.

If we evaluate the RHS in the context of the LHS, that evaluation might
change the LHS in ways that mess with being able to store the value later.
So for assignment or scalar values, evaluate the RHS first.

	* constexpr.c (cxx_eval_store_expression): Preevaluate scalar or
	assigned value.

From-SVN: r269003

gcc/cp/ChangeLog

@@ -1,5 +1,9 @@
 2019-02-18  Jason Merrill  <jason@redhat.com>
 
+	PR c++/89336 - multiple stores in constexpr stmt.
+	* constexpr.c (cxx_eval_store_expression): Preevaluate scalar or
+	assigned value.
+
 	* pt.c (check_explicit_specialization): If the declarator is a
 	template-id, only check whether the arguments are dependent.
 

gcc/cp/constexpr.c

@@ -3634,6 +3634,18 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
   maybe_simplify_trivial_copy (target, init);
 
   tree type = TREE_TYPE (target);
+  bool preeval = SCALAR_TYPE_P (type) || TREE_CODE (t) == MODIFY_EXPR;
+  if (preeval)
+    {
+      /* Evaluate the value to be stored without knowing what object it will be
+	 stored in, so that any side-effects happen first.  */
+      if (!SCALAR_TYPE_P (type))
+	new_ctx.ctor = new_ctx.object = NULL_TREE;
+      init = cxx_eval_constant_expression (&new_ctx, init, false,
+					   non_constant_p, overflow_p);
+      if (*non_constant_p)
+	return t;
+    }
   target = cxx_eval_constant_expression (ctx, target,
 					 true,
 					 non_constant_p, overflow_p);
@@ -3834,7 +3846,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
     }
   release_tree_vector (refs);
 
-  if (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type))
+  if (!preeval)
     {
       /* Create a new CONSTRUCTOR in case evaluation of the initializer
 	 wants to modify it.  */
@@ -3843,21 +3855,20 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  *valp = build_constructor (type, NULL);
 	  CONSTRUCTOR_NO_CLEARING (*valp) = no_zero_init;
 	}
-      else if (TREE_CODE (*valp) == PTRMEM_CST)
-	*valp = cplus_expand_constant (*valp);
       new_ctx.ctor = *valp;
       new_ctx.object = target;
+      init = cxx_eval_constant_expression (&new_ctx, init, false,
+					   non_constant_p, overflow_p);
+      if (target == object)
+	/* The hash table might have moved since the get earlier.  */
+	valp = ctx->values->get (object);
     }
 
-  init = cxx_eval_constant_expression (&new_ctx, init, false,
-				       non_constant_p, overflow_p);
   /* Don't share a CONSTRUCTOR that might be changed later.  */
   init = unshare_constructor (init);
-  if (target == object)
-    /* The hash table might have moved since the get earlier.  */
-    valp = ctx->values->get (object);
 
-  if (TREE_CODE (init) == CONSTRUCTOR)
+  if (*valp && TREE_CODE (*valp) == CONSTRUCTOR
+      && TREE_CODE (init) == CONSTRUCTOR)
     {
       /* An outer ctx->ctor might be pointing to *valp, so replace
 	 its contents.  */

gcc/testsuite/g++.dg/cpp1y/constexpr-89336-1.C

@@ -0,0 +1,35 @@
+// PR c++/89336
+// { dg-do compile { target c++14 } }
+
+template <typename T, int N> struct A {
+  T a[N];
+  constexpr T &operator[] (int x) { return a[x]; }
+  constexpr const T &operator[] (int x) const { return a[x]; }
+};
+
+constexpr A<int, 16>
+foo ()
+{
+  A<int, 16> r{};
+  for (int i = 0; i < 6; ++i)
+    r[i + 8] = r[i] = i + 1;
+  return r;
+}
+
+constexpr auto x = foo ();
+static_assert (x[0] == 1, "");
+static_assert (x[1] == 2, "");
+static_assert (x[2] == 3, "");
+static_assert (x[3] == 4, "");
+static_assert (x[4] == 5, "");
+static_assert (x[5] == 6, "");
+static_assert (x[6] == 0, "");
+static_assert (x[7] == 0, "");
+static_assert (x[8] == 1, "");
+static_assert (x[9] == 2, "");
+static_assert (x[10] == 3, "");
+static_assert (x[11] == 4, "");
+static_assert (x[12] == 5, "");
+static_assert (x[13] == 6, "");
+static_assert (x[14] == 0, "");
+static_assert (x[15] == 0, "");

gcc/testsuite/g++.dg/cpp1y/constexpr-89336-2.C

@@ -0,0 +1,56 @@
+// PR c++/89336
+// { dg-do compile { target c++14 } }
+
+constexpr int
+foo ()
+{
+  int a[16] = {};
+  int r = 0;
+  a[15] = a[14] = a[13] = a[12] = a[11] = a[10] = a[9] = a[8]
+    = a[7] = a[6] = a[5] = a[4] = a[3] = a[2] = a[1] = a[0] = 5;
+  for (int i = 0; i < 16; ++i)
+    r += a[i];
+  return r;
+}
+
+static_assert (foo () == 16 * 5, "");
+
+struct A { int a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p; };
+
+constexpr int
+bar ()
+{
+  A a {};
+  a.p = a.o = a.n = a.m = a.l = a.k = a.j = a.i
+    = a.h = a.g = a.f = a.e = a.d = a.c = a.b = a.a = 8;
+  return a.a + a.b + a.c + a.d + a.e + a.f + a.g + a.h
+	 + a.i + a.j + a.k + a.l + a.m + a.n + a.o + a.p;
+}
+
+static_assert (bar () == 16 * 8, "");
+
+constexpr int
+baz ()
+{
+  int a[16] = {};
+  int r = 0;
+  a[0] = a[1] = a[2] = a[3] = a[4] = a[5] = a[6] = a[7]
+    = a[8] = a[9] = a[10] = a[11] = a[12] = a[13] = a[14] = a[15] = 7;
+  for (int i = 0; i < 16; ++i)
+    r += a[i];
+  return r;
+}
+
+static_assert (baz () == 16 * 7, "");
+
+constexpr int
+qux ()
+{
+  A a {};
+  a.a = a.b = a.c = a.d = a.e = a.f = a.g = a.h
+    = a.i = a.j = a.k = a.l = a.m = a.n = a.o = a.p = 6;
+  return a.a + a.b + a.c + a.d + a.e + a.f + a.g + a.h
+	 + a.i + a.j + a.k + a.l + a.m + a.n + a.o + a.p;
+}
+
+static_assert (qux () == 16 * 6, "");