C++ PATCH for c++/85092, C++17 ICE with unused list constructor

[Jason Merrill]

Here, because the class has a list constructor, we avoid the shortcut
in build_special_member_call, so we need to use the new backup path
for C++17 copy elision in build_over_call, which means teaching
conv_binds_ref_to_prvalue about this case.

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

commit 053b17904db3de54265a6cda7dcd90390b0753b1
Author: Jason Merrill <ja...@redhat.com>
Date:   Tue Apr 3 14:10:44 2018 -0400

            PR c++/85092 - C++17 ICE with unused list constructor.
    
            * call.c (conv_binds_ref_to_prvalue): Also count ck_identity
            from a TARGET_EXPR.

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 9351918b23a..901f18c43e6 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -7611,6 +7611,9 @@ conv_binds_ref_to_prvalue (conversion *c)
     return true;
   if (c->kind == ck_user && TREE_CODE (c->type) != REFERENCE_TYPE)
     return true;
+  if (c->kind == ck_identity && c->u.expr
+      && TREE_CODE (c->u.expr) == TARGET_EXPR)
+    return true;
 
   return false;
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist99.C b/gcc/testsuite/g++.dg/cpp0x/initlist99.C
new file mode 100644
index 00000000000..d96f793d398
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist99.C
@@ -0,0 +1,13 @@
+// PR c++/85092
+// { dg-do compile { target c++11 } }
+
+#include <initializer_list>
+
+struct A
+{
+  A (std::initializer_list<char>);
+};
+
+A f ();
+
+A a { f () };