c++: CTAD with alias and nested template [PR105655]

Here, alias_ctad_tweaks expect tsubst_decl of a FUNCTION_DECL to return a
FUNCTION_DECL.  A reasonable expectation, but in this case we were replacing
the template args of the class-scope deduction guide with equivalent args,
so looking in the hash table we found the partial instantiation stored when
instantiating A<int>, which is a TEMPLATE_DECL.  It's fine for that to be
what is stored, but tsubst_function_decl should never return it.

	PR c++/105655

gcc/cp/ChangeLog:

	* pt.cc (build_template_decl): Add assert.
	(tsubst_function_decl): Don't return a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias13.C: New test.

2 files changed, 29 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 76913cb..021af01 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -5021,6 +5021,8 @@ maybe_update_decl_type (tree orig_type, tree scope)
 static tree
 build_template_decl (tree decl, tree parms, bool member_template_p)
 {
+  gcc_checking_assert (TREE_CODE (decl) != TEMPLATE_DECL);
+
   tree tmpl = build_lang_decl (TEMPLATE_DECL, DECL_NAME (decl), NULL_TREE);
   SET_DECL_LANGUAGE (tmpl, DECL_LANGUAGE (decl));
   DECL_TEMPLATE_PARMS (tmpl) = parms;
@@ -14074,7 +14076,9 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
 	{
 	  hash = hash_tmpl_and_args (gen_tmpl, argvec);
 	  if (tree spec = retrieve_specialization (gen_tmpl, argvec, hash))
-	    return spec;
+	    /* The spec for these args might be a partial instantiation of the
+	       template, but here what we want is the FUNCTION_DECL.  */
+	    return STRIP_TEMPLATE (spec);
 	}
     }
   else
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C
new file mode 100644
index 0000000..0a90a83
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C
@@ -0,0 +1,24 @@
+// PR c++/105655
+// { dg-do compile { target c++20 } }
+
+template <class T>
+struct A
+{
+  template <class L, class R>
+  struct B
+  {
+    B(const L & left, const R & right)
+    {}
+  };
+
+  template <class L, class R>
+  B(const L &, const R &) -> B<L, R>;
+};
+
+template <class L, class R>
+using C = A<int>::B<L, R>;
+
+int main()
+{
+  C x{0, 0};
+}