c++: CTAD for class tmpl defined inside partial spec [PR104294]

Here during deduction guide generation for the nested class template
B<char(int)>::C, the computation of outer_args yields the template
arguments relative to the primary template for B (i.e. {char(int)})
but what we really want is those relative to C's enclosing scope, the
partial specialization of B (i.e. {char, int}).

	PR c++/104294

gcc/cp/ChangeLog:

	* pt.cc (ctor_deduction_guides_for): Correct computation of
	outer_args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction106.C: New test.

2 files changed, 19 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 84d63f9181c..feee629f1dd 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -29563,7 +29563,9 @@ ctor_deduction_guides_for (tree tmpl, tsubst_flags_t complain)
   if (DECL_CLASS_SCOPE_P (tmpl)
       && CLASSTYPE_TEMPLATE_INSTANTIATION (DECL_CONTEXT (tmpl)))
     {
-      outer_args = CLASSTYPE_TI_ARGS (DECL_CONTEXT (tmpl));
+      outer_args = copy_node (CLASSTYPE_TI_ARGS (type));
+      gcc_assert (TMPL_ARGS_DEPTH (outer_args) > 1);
+      --TREE_VEC_LENGTH (outer_args);
       type = TREE_TYPE (most_general_template (tmpl));
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C
new file mode 100644
index 00000000000..b4c4d262f44
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C
@@ -0,0 +1,16 @@
+// PR c++/104294
+// { dg-do compile { target c++17 } }
+
+template<class> struct B;
+
+template<class R, class... Args>
+struct B<R(Args...)> {
+  template<class T> struct C { C(T); };
+  C(decltype(nullptr)) -> C<void*>;
+};
+
+using ty1 = decltype(B<char(int)>::C{0});
+using ty1 = B<char(int)>::C<int>;
+
+using ty2 = decltype(B<char(int)>::C{nullptr});
+using ty2 = B<char(int)>::C<void*>;