c++: CTAD and member alias template [PR102123]

When building a deduction guide from the Test constructor, we need to
rewrite the use of _dummy into a dependent reference, i.e. Test<T>::template
_dummy.  We were using SCOPE_REF for both type and non-type templates; we
need to use UNBOUND_CLASS_TEMPLATE for type templates.

	PR c++/102123

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy): Use make_unbound_class_template for rewriting
	a type template reference.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction110.C: New test.

2 files changed, 31 insertions, 0 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 41f1ef1..678063f 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -17021,6 +17021,9 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	      /* When rewriting a constructor into a deduction guide, a
 		 non-dependent name can become dependent, so memtmpl<args>
 		 becomes context::template memtmpl<args>.  */
+	      if (DECL_TYPE_TEMPLATE_P (t))
+		return make_unbound_class_template (context, DECL_NAME (t),
+						    NULL_TREE, complain);
 	      tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
 	      return build_qualified_name (type, context, DECL_NAME (t),
 					   /*template*/true);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction110.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction110.C
new file mode 100644
index 0000000..8eb5647
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction110.C
@@ -0,0 +1,28 @@
+// PR c++/102123
+// { dg-do compile { target c++17 } }
+
+template<template<typename...> typename Template, typename... Args>
+struct _dummy_forwarder {
+    using type = Template<Args...>;
+};
+
+template<template<typename...> typename Template, typename... Args>
+using dummy_forwarder = typename _dummy_forwarder<Template, Args...>::type;
+
+template<typename T>
+struct Test {
+    template<typename U> using _dummy = U;
+
+    using Element = dummy_forwarder<_dummy, T>;
+
+    Element _elem;
+
+    constexpr Test(const Element elem) : _elem(elem) { }
+};
+
+template<typename T>
+Test(T) -> Test<T>;
+
+void test() {
+    const auto t = Test(1);
+}