C++ PATCH for c++/68890 (ICE with constexpr value-initialization)

[Jason Merrill]

Here, my assertion that a CONSTRUCTOR should be empty when we start to 
give it an initial value was forgetting about the case of classes with 
non-user-defined constructors, where value-initialization first 
zero-initializes, then calls the synthesized constructor.

Tested x86_64-pc-linux-gnu, applying to trunk and 5.
commit c87b2db1a8bff1394c5e607f8d470f5eed20193c
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Feb 10 21:17:52 2016 -0500
    	PR c++/68990
    
    	* constexpr.c (verify_ctor_sanity): Remove CONSTRUCTOR_NELTS check.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 85fc64e..11037fb 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2202,7 +2202,8 @@ verify_ctor_sanity (const constexpr_ctx *ctx, tree type)
   gcc_assert (ctx->ctor);
   gcc_assert (same_type_ignoring_top_level_qualifiers_p
 	      (type, TREE_TYPE (ctx->ctor)));
-  gcc_assert (CONSTRUCTOR_NELTS (ctx->ctor) == 0);
+  /* We used to check that ctx->ctor was empty, but that isn't the case when
+     the object is zero-initialized before calling the constructor.  */
   if (ctx->object)
     gcc_assert (same_type_ignoring_top_level_qualifiers_p
 		(type, TREE_TYPE (ctx->object)));
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-value5.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-value5.C
new file mode 100644
index 0000000..8c67174
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-value5.C
@@ -0,0 +1,18 @@
+// PR c++/68990
+// { dg-do compile { target c++11 } }
+
+class ptr;
+template <long _Nm> struct A { typedef ptr _Type[_Nm]; };
+template <long _Nm> struct B { typename A<_Nm>::_Type _M_elems; };
+template <long N> class FixedVector : B<N> {
+public:
+  typedef B<1> base;
+  constexpr FixedVector() : base(), size_() {}
+  char size_;
+};
+class ptr {
+public:
+  constexpr ptr() : px_(){};
+  int px_;
+};
+FixedVector<1> a;