c++: alias CTAD and specializations table

A rewritten guide for alias CTAD isn't really a specialization of the
original guide, so we shouldn't register it as such.  This avoids an ICE
in the below modules testcase for which we otherwise crash due to the
guide's empty DECL_CONTEXT when walking the specializations table.  It
also preemptively avoids the same ICE in modules/concept-6 in C++23 mode
with the inherited CTAD patch.

gcc/cp/ChangeLog:

	* pt.cc (alias_ctad_tweaks): Pass use_spec_table=false to
	tsubst_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/concept-8.h: New test.
	* g++.dg/modules/concept-8_a.H: New test.
	* g++.dg/modules/concept-8_b.C: New test.

4 files changed, 29 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e9dcdb3..208fa21 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30020,7 +30020,8 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 	      /* Parms are to have DECL_CHAIN tsubsted, which would be skipped
 		 if cp_unevaluated_operand.  */
 	      cp_evaluated ev;
-	      g = tsubst_decl (DECL_TEMPLATE_RESULT (f), targs, complain);
+	      g = tsubst_decl (DECL_TEMPLATE_RESULT (f), targs, complain,
+			       /*use_spec_table=*/false);
 	    }
 	  if (g == error_mark_node)
 	    continue;
diff --git a/gcc/testsuite/g++.dg/modules/concept-8.h b/gcc/testsuite/g++.dg/modules/concept-8.h
new file mode 100644
index 0000000..a25f9b7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/concept-8.h
@@ -0,0 +1,14 @@
+// A version of concept-6.h using an alias template + alias CTAD
+
+template<typename _Callable>
+struct Base
+{
+  Base (const _Callable &)
+    requires true
+  {}
+};
+
+template<typename _Callable> requires true
+using Derived = Base<_Callable>;
+
+inline Derived all = [] (auto&& __r) {};
diff --git a/gcc/testsuite/g++.dg/modules/concept-8_a.H b/gcc/testsuite/g++.dg/modules/concept-8_a.H
new file mode 100644
index 0000000..da04677
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/concept-8_a.H
@@ -0,0 +1,5 @@
+// { dg-require-effective-target c++20 }
+// { dg-additional-options "-fmodule-header -fconcepts" }
+// { dg-module-cmi {} }
+
+#include "concept-8.h"
diff --git a/gcc/testsuite/g++.dg/modules/concept-8_b.C b/gcc/testsuite/g++.dg/modules/concept-8_b.C
new file mode 100644
index 0000000..9a9f014
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/concept-8_b.C
@@ -0,0 +1,8 @@
+// { dg-require-effective-target c++20 }
+// { dg-additional-options "-fmodules-ts -fconcepts -fdump-lang-module-alias -fno-module-lazy" }
+
+#include "concept-8.h"
+import "concept-8_a.H";
+
+// { dg-final { scan-lang-dump-times {named merge key \(matched\) function_decl:'::Base<::._anon_0>::__ct '} 2 module } }
+// { dg-final { scan-lang-dump-not {merge key \(new\)} module } }