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

[Patrick Palka via Gcc-patches]

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

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

        PR c++/104294

gcc/cp/ChangeLog:

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

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1z/class-deduction106.C: New test.
---
 gcc/cp/pt.cc                                  |  4 +++-
 .../g++.dg/cpp1z/class-deduction106.C         | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction106.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index f46a7ad6655..0fe0ad4e44e 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -29568,7 +29568,9 @@ 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_HAVE_MULTIPLE_LEVELS (outer_args));
+      --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..382f6b70ef5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C
@@ -0,0 +1,19 @@
+// 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*>;
-- 
2.35.0