C++ PATCH for c++/83835, C++17 error with constexpr constructor

[Jason Merrill]

The C++17 path in build_special_member_call converts to the desired
type and then builds an INIT_EXPR to initialize the object.  Since
this does not involve actually creating a temporary, we should set
TARGET_EXPR_DIRECT_INIT_P.

Tested x86_64-pc-linux-gnu, applying to trunk.

commit f2cf58eab564e32201c403f292da3ff99385d48e
Author: Jason Merrill <ja...@redhat.com>
Date:   Fri Feb 16 14:47:34 2018 -0500

            PR c++/83835 - C++17 error with constructor ctors.
    
            * call.c (build_special_member_call): Set TARGET_EXPR_DIRECT_INIT_P.

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 5698ff60a4d..d3d0966f65c 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8834,6 +8834,9 @@ build_special_member_call (tree instance, tree name, 
vec<tree, va_gc> **args,
        {
          if (is_dummy_object (instance))
            return arg;
+         else if (TREE_CODE (arg) == TARGET_EXPR)
+           TARGET_EXPR_DIRECT_INIT_P (arg) = true;
+
          if ((complain & tf_error)
              && (flags & LOOKUP_DELEGATING_CONS))
            check_self_delegation (arg);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor21.C 
b/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor21.C
new file mode 100644
index 00000000000..4bf57b9e897
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor21.C
@@ -0,0 +1,15 @@
+// PR c++/83835
+// { dg-do compile { target c++11 } }
+
+struct Z
+{
+  void const * p_;
+  constexpr Z( void const * p ): p_( p ) {}
+  ~Z();
+};
+
+struct Y
+{
+  Z z_;
+  constexpr Y() noexcept: z_( this ) {}
+};