C++ PATCH for c++/66450 (wrong code with constexpr forwarding constructor)

[Jason Merrill]

The problem here was that cxx_eval_store_expression was storing the 
value of the target variable in the hash table, but then 
cxx_eval_call_expression was looking at ctx->ctor to find the value, and 
it wasn't there.  Fixed by handling full-object stores by swapping out 
CONSTRUCTOR_ELTS.

Tested x86_64-pc-linux-gnu, applying to trunk and 5.
commit 4664b3a0104705062119cd1477fc23b5584a83e5
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Jun 10 15:07:31 2015 -0400
    	PR c++/66450
    	* constexpr.c (cxx_eval_store_expression): Avoid messing up outer
    	ctx->ctor.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 8fa2eb3..af6b39e 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2671,11 +2671,13 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
     }
   release_tree_vector (refs);
 
-  if ((AGGREGATE_TYPE_P (TREE_TYPE (t)) || VECTOR_TYPE_P (TREE_TYPE (t))))
+  if (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type))
     {
       /* Create a new CONSTRUCTOR in case evaluation of the initializer
 	 wants to modify it.  */
-      *valp = new_ctx.ctor = build_constructor (TREE_TYPE (t), NULL);
+      new_ctx.ctor = build_constructor (type, NULL);
+      if (*valp == NULL_TREE)
+	*valp = new_ctx.ctor;
       CONSTRUCTOR_NO_IMPLICIT_ZERO (new_ctx.ctor) = true;
       new_ctx.object = target;
     }
@@ -2683,8 +2685,16 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
   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.  */
-    ctx->values->put (object, init);
+    {
+      /* The hash table might have moved since the get earlier.  */
+      valp = ctx->values->get (object);
+      if (TREE_CODE (init) == CONSTRUCTOR)
+	/* An outer ctx->ctor might be pointing to *valp, so just replace
+	   its contents.  */
+	CONSTRUCTOR_ELTS (*valp) = CONSTRUCTOR_ELTS (init);
+      else
+	*valp = init;
+    }
   else
     *valp = init;
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-fwctor1.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-fwctor1.C
new file mode 100644
index 0000000..d25c9c7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-fwctor1.C
@@ -0,0 +1,19 @@
+// PR c++/66450
+// { dg-do compile { target c++11 } }
+
+struct foo {
+  constexpr foo(int a);
+  constexpr foo(int a, int b, int c): a{a}, b{b}, c{c} {}
+
+  int a, b, c;
+};
+
+constexpr foo make_foo(int a) { return foo{a, a+1, a+2}; }
+constexpr foo::foo(int a): foo{make_foo(a)} {}
+
+int main() {
+  constexpr const foo f{3};
+  static_assert(f.a == 3, "");
+  static_assert(f.b == 4, "");
+  static_assert(f.c == 5, "");
+}