C++ PATCH for c++/82461, member list-initialization and -std=c++17

[Jason Merrill]

In C++17 mode, build_special_member_call builds up an initializer for
the object and then uses it directly rather than pass it to the copy
constructor.  This pattern results in a TARGET_EXPR showing up in
potential_constant_expression, which complains about a TARGET_EXPR
indicating a temporary of non-literal type, which it isn't in this
case because it's being used to initialize a non-temporary object.
This patch recognizes that case by checking TARGET_EXPR_DIRECT_INIT_P.

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

commit 9084980615c1aa29277fee9fe33a0b52bbc0ab94
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Jan 17 17:24:16 2018 -0500

            PR c++/82461 - constexpr list-initialized member
    
            * constexpr.c (potential_constant_expression_1): Check
            TARGET_EXPR_DIRECT_INIT_P.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 8984613aa41..9a548d29bbc 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5726,7 +5726,8 @@ potential_constant_expression_1 (tree t, bool want_rval, 
bool strict, bool now,
       return RECUR (TREE_OPERAND (t, 1), want_rval);
 
     case TARGET_EXPR:
-      if (!literal_type_p (TREE_TYPE (t)))
+      if (!TARGET_EXPR_DIRECT_INIT_P (t)
+         && !literal_type_p (TREE_TYPE (t)))
        {
          if (flags & tf_error)
            {
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-list2.C 
b/gcc/testsuite/g++.dg/cpp0x/constexpr-list2.C
new file mode 100644
index 00000000000..780a64dbbc4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-list2.C
@@ -0,0 +1,20 @@
+// PR c++/82461
+// { dg-do compile { target c++11 } }
+
+class A {
+private:
+public:
+  constexpr A() {}
+  ~A() {}
+};
+
+class B {
+private:
+  A  a;
+public:
+  constexpr B() : a{} {}
+// works
+// constexpr B() : a() {}
+
+  ~B() {}
+};