c++: implicit deduction guides, protected access

Jonathan raised this issue with CWG, and there seems to be general agreement
that a deduction guide generated from a constructor should have access to
the same names that the constructor has access to.  That seems to be as easy
as setting DECL_CONTEXT.

gcc/cp/ChangeLog:

	* pt.c (build_deduction_guide): Treat the implicit deduction guide
	as a member of the class.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction-access1.C: New test.
	* g++.dg/cpp1z/class-deduction-access2.C: New test.

3 files changed, 31 insertions, 0 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 23d2623..32cd0b7 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28803,6 +28803,9 @@ build_deduction_guide (tree type, tree ctor, tree outer_args, tsubst_flags_t com
     DECL_ABSTRACT_ORIGIN (ded_tmpl) = fn_tmpl;
   if (ci)
     set_constraints (ded_tmpl, ci);
+  /* The artificial deduction guide should have same access as the
+     constructor.  */
+  DECL_CONTEXT (ded_fn) = type;
 
   return ded_tmpl;
 }
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction-access1.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access1.C
new file mode 100644
index 0000000..2424abb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access1.C
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++17 } }
+
+template<typename T>
+struct Base
+{
+protected:
+  using type = T;
+};
+
+template<typename T>
+struct Cont : Base<T>
+{
+  using argument_type = typename Base<T>::type;
+
+  Cont(T, argument_type) { }
+};
+
+Cont c(1, 1);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction-access2.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access2.C
new file mode 100644
index 0000000..87f2031
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access2.C
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++17 } }
+
+struct B {
+protected:
+    struct type {};
+};
+template<typename T> struct D : B {
+    D(T, typename T::type);
+};
+D c = {B(), {}};