c++: reference list-init, conversion fn [PR113141]

The original testcase in PR113141 is an instance of CWG1996; the standard
fails to consider conversion functions when initializing a reference
directly from an initializer-list of one element, but then does consider
them when initializing a temporary.  I have a proposed fix for this defect,
which is implemented here.

	DR 1996
	PR c++/113141

gcc/cp/ChangeLog:

	* call.cc (reference_binding): Check direct binding from
	a single-element list.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist-ref1.C: New test.
	* g++.dg/cpp0x/initlist-ref2.C: New test.
	* g++.dg/cpp0x/initlist-ref3.C: New test.

Co-authored-by: Patrick Palka <ppalka@redhat.com>

3 files changed, 39 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-ref1.C b/gcc/testsuite/g++.dg/cpp0x/initlist-ref1.C
new file mode 100644
index 0000000..f893f12
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-ref1.C
@@ -0,0 +1,16 @@
+// PR c++/113141
+// { dg-do compile { target c++11 } }
+
+struct ConvToRef {
+  operator int&();
+};
+
+struct A { int& r; };
+
+void f(A);
+
+int main() {
+  ConvToRef c;
+  A a{{c}};
+  f({{c}});
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-ref2.C b/gcc/testsuite/g++.dg/cpp0x/initlist-ref2.C
new file mode 100644
index 0000000..401d868
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-ref2.C
@@ -0,0 +1,10 @@
+// CWG1996
+// { dg-do compile { target c++11 } }
+
+struct S { operator struct D &(); } s;
+D &d{s};			// OK, direct binding
+
+namespace N1 {
+  struct S { operator volatile struct D &(); } s;
+  const D &dr{s};    // { dg-error "invalid user-defined|discards qualifiers" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-ref3.C b/gcc/testsuite/g++.dg/cpp0x/initlist-ref3.C
new file mode 100644
index 0000000..e2cc1de
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-ref3.C
@@ -0,0 +1,13 @@
+// CWG1996
+// { dg-do compile { target c++11 } }
+
+struct D { constexpr D() {} } d;
+struct S {
+  template <class T>
+  constexpr operator T& () const { return d; }
+};
+constexpr S s;
+constexpr const D &dr1(s);
+static_assert (&dr1 == &d, "");
+constexpr const D &dr2{s};
+static_assert (&dr2 == &d, "");