Re: [PATCH] c++/modules: member alias tmpl partial inst [PR103994]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 7 Mar 2024, Patrick Palka wrote:

> On Wed, 6 Mar 2024, Jason Merrill wrote:
> 
> > On 3/4/24 17:26, Patrick Palka wrote:
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > > OK for trunk?
> > > 
> > > -- >8 --
> > > 
> > > Alias templates are weird in that their specializations can appear in
> > > both decl_specializations and type_specializations.  They appear in the
> > > latter only at parse time via finish_template_type.  This should probably
> > > be revisited in GCC 15 since it seems sufficient to store them only in
> > > decl_specializations.
> > 
> > It looks like most all of lookup_template_class is wrong for alias templates.
> > 
> > Can we move the alias template handling up higher and unconditionally return
> > the result of tsubst?
> 
> This works nicely (although we have to use instantiate_alias_template
> directly instead of tsubst since tsubst would first substitute the
> uncoerced arguments into the generic DECL_TI_ARGS which breaks for
> for parameter packs).  And it allows for some nice simplifications in
> the modules code which had to handle alias template specializations
> specially.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu.
> 
> -- >8 --
> 
> Subject: [PATCH] c++/modules: member alias tmpl partial inst [PR103994]
> 
> Alias templates are weird in that their specializations can appear in
> both decl_specializations and type_specializations.  They appear in the
> type table only at parse time via finish_template_type.  There seems
> to be no good reason for this, and the code paths end up stepping over
> each other in particular for a partial alias template instantiation such
> as A<B>::key_arg<T> in the below modules testcase: the type code path
> (lookup_template_class) wants to set TI_TEMPLATE to the most general
> template whereas the decl code path (tsubst_template_decl called during
> instantiation of A<B>) already set TI_TEMPLATE to the partially
> instantiated TEMPLATE_DECL.  This ends up confusing modules which
> decides to stream the logically equivalent TYPE_DECL and TEMPLATE_DECL
> for this partial alias template instantiation separately.
> 
> This patch fixes this by making lookup_template_class dispatch to
> instantiatie_alias_template early for alias template specializations.
> In turn we now only add such specializations to the decl table and
> not also the type table.  This admits some nice simplification in
> the modules code which otherwise has to cope with such specializations
> appearing in both tables.
> 
> 	PR c++/103994
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (add_mergeable_specialization): Remove is_alias
> 	parameter.
> 	* module.cc (depset::disc_bits::DB_ALIAS_SPEC_BIT): Remove.
> 	(depset::is_alias): Remove.
> 	(merge_kind::MK_tmpl_alias_mask): Remove.
> 	(merge_kind::MK_alias_spec): Remove.
> 	(merge_kind_name): Remove entries for alias specializations.
> 	(trees_in::decl_value): Adjust add_mergeable_specialization
> 	calls.
> 	(trees_out::get_merge_kind) <case depset::EK_SPECIALIZATION>:
> 	Use MK_decl_spec for alias template specializations.
> 	(trees_out::key_mergeable): Simplify after MK_tmpl_alias_mask
> 	removal.
> 	(specialization_add): Don't allow alias templates when !decl_p.
> 	(depset::hash::add_specializations): Remove now-dead code
> 	accomodating alias template specializations in the type table.
> 	* pt.cc (lookup_template_class): Dispatch early to
> 	instantiate_alias_template for alias templates.  Simplify
> 	accordingly.
> 	(add_mergeable_specialization): Remove alias_p parameter and
> 	simplify accordingly.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/pr99425-1_b.H: s/alias/decl in dump scan.
> 	* g++.dg/modules/tpl-alias-1_a.H: Likewise.
> 	* g++.dg/modules/tpl-alias-2_a.H: New test.
> 	* g++.dg/modules/tpl-alias-2_b.C: New test.
> ---
>  gcc/cp/cp-tree.h                             |  3 +-
>  gcc/cp/module.cc                             | 50 ++----------
>  gcc/cp/pt.cc                                 | 84 ++++++++------------
>  gcc/testsuite/g++.dg/modules/pr99425-1_b.H   |  2 +-
>  gcc/testsuite/g++.dg/modules/tpl-alias-1_a.H |  2 +-
>  gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H | 15 ++++
>  gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C |  9 +++
>  7 files changed, 69 insertions(+), 96 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H
>  create mode 100644 gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 4469d965ef0..14895bc6585 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7642,8 +7642,7 @@ extern void walk_specializations		(bool,
>  						 void *);
>  extern tree match_mergeable_specialization	(bool is_decl, spec_entry *);
>  extern unsigned get_mergeable_specialization_flags (tree tmpl, tree spec);
> -extern void add_mergeable_specialization        (bool is_decl, bool is_alias,
> -						 spec_entry *,
> +extern void add_mergeable_specialization        (bool is_decl, spec_entry *,
>  						 tree outer, unsigned);
>  extern tree add_to_template_args		(tree, tree);
>  extern tree add_outermost_template_args		(tree, tree);
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index 4ecd6d7c813..cc038ee9cba 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -2306,10 +2306,7 @@ private:
>      /* The following bits are not independent, but enumerating them is
>         awkward.  */
>      DB_ALIAS_TMPL_INST_BIT,	/* An alias template instantiation. */

It seems we don't need DB_ALIAS_TMPL_INST_BIT anymore either, so the
updated patch below removes it too.

-- >8 --

	PR c++/103994

gcc/cp/ChangeLog:

	* cp-tree.h (add_mergeable_specialization): Remove is_alias
	parameter.
	* module.cc (depset::disc_bits::DB_ALIAS_TMPL_INST_BIT): Remove.
	(depset::disc_bits::DB_ALIAS_SPEC_BIT): Remove.
	(depset::is_alias_tmpl_inst): Remove.
	(depset::is_alias): Remove.
	(merge_kind::MK_tmpl_alias_mask): Remove.
	(merge_kind::MK_alias_spec): Remove.
	(merge_kind_name): Remove entries for alias specializations.
	(trees_out::core_vals) <case TEMPLATE_DECL>: Adjust after
	removing is_alias_tmpl_inst.
	(trees_in::decl_value): Adjust add_mergeable_specialization
	calls.
	(trees_out::get_merge_kind) <case depset::EK_SPECIALIZATION>:
	Use MK_decl_spec for alias template specializations.
	(trees_out::key_mergeable): Simplify after MK_tmpl_alias_mask
	removal.
	(depset::hash::make_dependency): Adjust after removing
	DB_ALIAS_TMPL_INST_BIT.
	(specialization_add): Don't allow alias templates when !decl_p.
	(depset::hash::add_specializations): Remove now-dead code
	accomodating alias template specializations in the type table.
	* pt.cc (lookup_template_class): Dispatch early to
	instantiate_alias_template for alias templates.  Simplify
	accordingly.
	(add_mergeable_specialization): Remove alias_p parameter and
	simplify accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr99425-1_b.H: s/alias/decl in dump scan.
	* g++.dg/modules/tpl-alias-1_a.H: Likewise.
	* g++.dg/modules/tpl-alias-2_a.H: New test.
	* g++.dg/modules/tpl-alias-2_b.C: New test.
---
 gcc/cp/cp-tree.h                             |  3 +-
 gcc/cp/module.cc                             | 86 ++++----------------
 gcc/cp/pt.cc                                 | 84 ++++++++-----------
 gcc/testsuite/g++.dg/modules/pr99425-1_b.H   |  2 +-
 gcc/testsuite/g++.dg/modules/tpl-alias-1_a.H |  2 +-
 gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H | 15 ++++
 gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C |  9 ++
 7 files changed, 78 insertions(+), 123 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H
 create mode 100644 gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 4469d965ef0..14895bc6585 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7642,8 +7642,7 @@ extern void walk_specializations		(bool,
 						 void *);
 extern tree match_mergeable_specialization	(bool is_decl, spec_entry *);
 extern unsigned get_mergeable_specialization_flags (tree tmpl, tree spec);
-extern void add_mergeable_specialization        (bool is_decl, bool is_alias,
-						 spec_entry *,
+extern void add_mergeable_specialization        (bool is_decl, spec_entry *,
 						 tree outer, unsigned);
 extern tree add_to_template_args		(tree, tree);
 extern tree add_outermost_template_args		(tree, tree);
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 4ecd6d7c813..dfe4536c0ae 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -2305,11 +2305,7 @@ private:
     DB_HIDDEN_BIT,		/* A hidden binding.  */
     /* The following bits are not independent, but enumerating them is
        awkward.  */
-    DB_ALIAS_TMPL_INST_BIT,	/* An alias template instantiation. */
-    DB_ALIAS_SPEC_BIT,		/* Specialization of an alias template
-				   (in both spec tables).  */
-    DB_TYPE_SPEC_BIT,		/* Specialization in the type table.
-				   */
+    DB_TYPE_SPEC_BIT,		/* Specialization in the type table.  */
     DB_FRIEND_SPEC_BIT,		/* An instantiated template friend.  */
   };
 
@@ -2400,14 +2396,6 @@ public:
   {
     return get_flag_bit<DB_UNREACHED_BIT> ();
   }
-  bool is_alias_tmpl_inst () const
-  {
-    return get_flag_bit<DB_ALIAS_TMPL_INST_BIT> ();
-  }
-  bool is_alias () const
-  {
-    return get_flag_bit<DB_ALIAS_SPEC_BIT> ();
-  }
   bool is_hidden () const
   {
     return get_flag_bit<DB_HIDDEN_BIT> ();
@@ -2782,13 +2770,11 @@ enum merge_kind
   MK_template_mask = 0x10,  /* A template specialization.  */
 
   MK_tmpl_decl_mask = 0x4, /* In decl table.  */
-  MK_tmpl_alias_mask = 0x2, /* Also in type table  */
 
   MK_tmpl_tmpl_mask = 0x1, /* We want TEMPLATE_DECL.  */
 
   MK_type_spec = MK_template_mask,
   MK_decl_spec = MK_template_mask | MK_tmpl_decl_mask,
-  MK_alias_spec = MK_decl_spec | MK_tmpl_alias_mask,
 
   MK_hwm = 0x20
 };
@@ -2806,7 +2792,7 @@ static char const *const merge_kind_name[MK_hwm] =
     NULL, NULL,
 
     "decl spec", "decl tmpl spec",	/* 20,21 decl (template).  */
-    "alias spec", "alias tmpl spec",	/* 22,23 alias (template). */
+    NULL, NULL,
     NULL, NULL, NULL, NULL,
     NULL, NULL, NULL, NULL,
   };
@@ -6356,8 +6342,7 @@ trees_out::core_vals (tree t)
       gcc_checking_assert
       	(TREE_VISITED (((lang_tree_node *)t)->template_decl.arguments));
       gcc_checking_assert
-	(TREE_VISITED (((lang_tree_node *)t)->template_decl.result)
-	 || dep_hash->find_dependency (t)->is_alias_tmpl_inst ());
+	(TREE_VISITED (((lang_tree_node *)t)->template_decl.result));
       if (DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P (t))
 	WT (DECL_CHAIN (t));
       break;
@@ -8306,16 +8291,13 @@ trees_in::decl_value ()
 	{
 	  bool is_type = TREE_CODE (inner) == TYPE_DECL;
 	  spec.spec = is_type ? type : inner;
-	  add_mergeable_specialization (!is_type, false,
-					&spec, decl, spec_flags);
+	  add_mergeable_specialization (!is_type, &spec, decl, spec_flags);
 	}
       else if (mk & MK_template_mask)
 	{
 	  bool is_type = !(mk & MK_tmpl_decl_mask);
 	  spec.spec = is_type ? type : mk & MK_tmpl_tmpl_mask ? inner : decl;
-	  add_mergeable_specialization (!is_type,
-					!is_type && mk & MK_tmpl_alias_mask,
-					&spec, decl, spec_flags);
+	  add_mergeable_specialization (!is_type, &spec, decl, spec_flags);
 	}
 
       if (NAMESPACE_SCOPE_P (decl)
@@ -8392,7 +8374,7 @@ trees_in::decl_value ()
       if (!e)
 	{
 	  spec.spec = inner;
-	  add_mergeable_specialization (true, false, &spec, decl, spec_flags);
+	  add_mergeable_specialization (true, &spec, decl, spec_flags);
 	}
       else if (e != existing)
 	set_overrun ();
@@ -10620,8 +10602,6 @@ trees_out::get_merge_kind (tree decl, depset *dep)
 	  mk = MK_friend_spec;
 	else if (dep->is_type_spec ())
 	  mk = MK_type_spec;
-	else if (dep->is_alias ())
-	  mk = MK_alias_spec;
 	else
 	  mk = MK_decl_spec;
 
@@ -10732,11 +10712,6 @@ trees_out::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
 	  gcc_assert (existing);
 	  if (mk & MK_tmpl_decl_mask)
 	    {
-	      if (mk & MK_tmpl_alias_mask)
-		/* It should be in both tables.  */
-		gcc_checking_assert
-		  (same_type_p (match_mergeable_specialization (false, entry),
-				TREE_TYPE (existing)));
 	      if (mk & MK_tmpl_tmpl_mask)
 		existing = DECL_TI_TEMPLATE (existing);
 	    }
@@ -12830,17 +12805,12 @@ depset::hash::make_dependency (tree decl, entity_kind ek)
 	 bindings.  */
       *slot = dep = make_entity (decl, ek, has_def);
 
-      if (TREE_CODE (decl) == TEMPLATE_DECL)
-	{
-	  if (DECL_ALIAS_TEMPLATE_P (decl) && DECL_TEMPLATE_INFO (decl))
-	    dep->set_flag_bit<DB_ALIAS_TMPL_INST_BIT> ();
-	  else if (CHECKING_P)
-	    /* The template_result should otherwise not be in the
-	       table, or be an empty redirect (created above).  */
-	    if (auto *eslot = entity_slot (DECL_TEMPLATE_RESULT (decl), false))
-	      gcc_checking_assert ((*eslot)->get_entity_kind () == EK_REDIRECT
-				   && !(*eslot)->deps.length ());
-	}
+      if (CHECKING_P && TREE_CODE (decl) == TEMPLATE_DECL)
+	/* The template_result should otherwise not be in the
+	   table, or be an empty redirect (created above).  */
+	if (auto *eslot = entity_slot (DECL_TEMPLATE_RESULT (decl), false))
+	  gcc_checking_assert ((*eslot)->get_entity_kind () == EK_REDIRECT
+			       && !(*eslot)->deps.length ());
 
       if (ek != EK_USING)
 	{
@@ -13228,16 +13198,11 @@ specialization_add (bool decl_p, spec_entry *entry, void *data_)
 	 heuristic.  We don't attempt to replicate that algorithm, but
 	 observe its behaviour and reproduce it upon read back.  */
 
-       gcc_checking_assert (DECL_ALIAS_TEMPLATE_P (entry->tmpl)
-			   || TREE_CODE (entry->spec) == ENUMERAL_TYPE
+       gcc_checking_assert (TREE_CODE (entry->spec) == ENUMERAL_TYPE
 			   || DECL_CLASS_TEMPLATE_P (entry->tmpl));
 
-       /* Only alias templates can appear in both tables (and
-	  if they're in the type table they must also be in the decl
-	  table).  */
        gcc_checking_assert
-	 (!match_mergeable_specialization (true, entry)
-	  == !DECL_ALIAS_TEMPLATE_P (entry->tmpl));
+	 (!match_mergeable_specialization (true, entry));
     }
   else if (VAR_OR_FUNCTION_DECL_P (entry->spec))
     gcc_checking_assert (!DECL_LOCAL_DECL_P (entry->spec));
@@ -13293,7 +13258,6 @@ depset::hash::add_specializations (bool decl_p)
       spec_entry *entry = data.pop ();
       tree spec = entry->spec;
       int use_tpl = 0;
-      bool is_alias = false;
       bool is_friend = false;
 
       if (decl_p && DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P (entry->tmpl))
@@ -13301,13 +13265,7 @@ depset::hash::add_specializations (bool decl_p)
 	   instantiation.  */
 	is_friend = true;
 
-      if (!decl_p && DECL_ALIAS_TEMPLATE_P (entry->tmpl))
-	{
-	  spec = TYPE_NAME (spec);
-	  is_alias = true;
-	}
-
-      if (decl_p || is_alias)
+      if (decl_p)
 	{
 	  if (tree ti = DECL_TEMPLATE_INFO (spec))
 	    {
@@ -13392,20 +13350,10 @@ depset::hash::add_specializations (bool decl_p)
       gcc_checking_assert (!TREE_VISITED (spec));
       depset *dep = make_dependency (spec, depset::EK_SPECIALIZATION);
       if (dep->is_special ())
-	{
-	  /* An already located specialization, this must be the TYPE
-	     corresponding to an alias_decl we found in the decl
-	     table.  */
-	  spec_entry *other = reinterpret_cast <spec_entry *> (dep->deps[0]);
-	  gcc_checking_assert (!decl_p && is_alias && !dep->is_type_spec ());
-	  gcc_checking_assert (other->tmpl == entry->tmpl
-			       && template_args_equal (other->args, entry->args)
-			       && TREE_TYPE (other->spec) == entry->spec);
-	  dep->set_flag_bit<DB_ALIAS_SPEC_BIT> ();
-	}
+	gcc_unreachable ();
       else
 	{
-	  gcc_checking_assert (decl_p || !is_alias);
+	  gcc_checking_assert (decl_p);
 	  if (dep->get_entity_kind () == depset::EK_REDIRECT)
 	    dep = dep->deps[0];
 	  else if (dep->get_entity_kind () == depset::EK_SPECIALIZATION)
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d73f6d93485..99aa64b1cc3 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10055,6 +10055,32 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
       /* Now we should have enough arguments.  */
       gcc_assert (parm_depth == arg_depth);
 
+      if (DECL_ALIAS_TEMPLATE_P (gen_tmpl))
+	{
+	  /* The user referred to a specialization of an alias
+	    template represented by GEN_TMPL.
+
+	    [temp.alias]/2 says:
+
+		When a template-id refers to the specialization of an
+		alias template, it is equivalent to the associated
+		type obtained by substitution of its
+		template-arguments for the template-parameters in the
+		type-id of the alias template.  */
+
+	  t = instantiate_alias_template (gen_tmpl, arglist, complain);
+	  /* Note that the call above (by indirectly calling
+	     register_specialization in tsubst_decl) registers the
+	     TYPE_DECL representing the specialization of the alias
+	     template.  So next time someone substitutes ARGLIST for
+	     the template parms into the alias template (GEN_TMPL),
+	     she'll get that TYPE_DECL back.  */
+
+	  if (t == error_mark_node)
+	    return error_mark_node;
+	  return TREE_TYPE (t);
+	}
+
       /* From here on, we're only interested in the most general
 	 template.  */
 
@@ -10120,7 +10146,6 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
          lookup. This prevents redundant checks on previously
          instantiated specializations. */
       if (flag_concepts
-	  && !DECL_ALIAS_TEMPLATE_P (gen_tmpl)
 	  && !constraints_satisfied_p (gen_tmpl, arglist))
         {
           if (complain & tf_error)
@@ -10189,31 +10214,7 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
 	context = global_namespace;
 
       /* Create the type.  */
-      if (DECL_ALIAS_TEMPLATE_P (gen_tmpl))
-	{
-	  /* The user referred to a specialization of an alias
-	    template represented by GEN_TMPL.
-
-	    [temp.alias]/2 says:
-
-	        When a template-id refers to the specialization of an
-		alias template, it is equivalent to the associated
-		type obtained by substitution of its
-		template-arguments for the template-parameters in the
-		type-id of the alias template.  */
-
-	  t = tsubst (TREE_TYPE (gen_tmpl), arglist, complain, in_decl);
-	  /* Note that the call above (by indirectly calling
-	     register_specialization in tsubst_decl) registers the
-	     TYPE_DECL representing the specialization of the alias
-	     template.  So next time someone substitutes ARGLIST for
-	     the template parms into the alias template (GEN_TMPL),
-	     she'll get that TYPE_DECL back.  */
-
-	  if (t == error_mark_node)
-	    return t;
-	}
-      else if (TREE_CODE (template_type) == ENUMERAL_TYPE)
+      if (TREE_CODE (template_type) == ENUMERAL_TYPE)
 	{
 	  if (!is_dependent_type)
 	    {
@@ -10301,8 +10302,7 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
 	    }
 	}
 
-      if (OVERLOAD_TYPE_P (t)
-	  && !DECL_ALIAS_TEMPLATE_P (gen_tmpl))
+      if (OVERLOAD_TYPE_P (t))
 	{
 	  static const char *tags[] = {"abi_tag", "may_alias"};
 
@@ -10369,7 +10369,7 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
 	{
 	  TREE_VEC_LENGTH (arglist)--;
 	  ++processing_template_decl;
-	  tree tinfo = TYPE_TEMPLATE_INFO_MAYBE_ALIAS (TREE_TYPE (gen_tmpl));
+	  tree tinfo = TYPE_TEMPLATE_INFO (TREE_TYPE (gen_tmpl));
 	  tree partial_inst_args =
 	    tsubst (INNERMOST_TEMPLATE_ARGS (TI_ARGS (tinfo)),
 		    arglist, complain, NULL_TREE);
@@ -10407,17 +10407,9 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
 	     TEMPLATE_PARM_LEVEL.  */
 	  found = tsubst (gen_tmpl, arglist, tf_none, NULL_TREE);
 	  TREE_VEC_LENGTH (arglist)++;
-	  /* FOUND is either a proper class type, or an alias
-	     template specialization.  In the later case, it's a
-	     TYPE_DECL, resulting from the substituting of arguments
-	     for parameters in the TYPE_DECL of the alias template
-	     done earlier.  So be careful while getting the template
-	     of FOUND.  */
 	  found = (TREE_CODE (found) == TEMPLATE_DECL
 		   ? found
-		   : (TREE_CODE (found) == TYPE_DECL
-		      ? DECL_TI_TEMPLATE (found)
-		      : CLASSTYPE_TI_TEMPLATE (found)));
+		   : CLASSTYPE_TI_TEMPLATE (found));
 
 	  if (DECL_CLASS_TEMPLATE_P (found)
 	      && CLASSTYPE_TEMPLATE_SPECIALIZATION (TREE_TYPE (found)))
@@ -10447,8 +10439,7 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
 		     DECL_TEMPLATE_INSTANTIATIONS (found));
 
       if (TREE_CODE (template_type) == ENUMERAL_TYPE
-	  && !uses_template_parms (current_nonlambda_scope ())
-	  && !DECL_ALIAS_TEMPLATE_P (gen_tmpl))
+	  && !uses_template_parms (current_nonlambda_scope ()))
 	/* Now that the type has been registered on the instantiations
 	   list, we set up the enumerators.  Because the enumeration
 	   constants may involve the enumeration type itself, we make
@@ -31578,8 +31569,8 @@ get_mergeable_specialization_flags (tree tmpl, tree decl)
    get_mergeable_specialization_flags.  */
 
 void
-add_mergeable_specialization (bool decl_p, bool alias_p, spec_entry *elt,
-			      tree decl, unsigned flags)
+add_mergeable_specialization (bool decl_p, spec_entry *elt, tree decl,
+			      unsigned flags)
 {
   hashval_t hash = spec_hasher::hash (elt);
   if (decl_p)
@@ -31590,15 +31581,8 @@ add_mergeable_specialization (bool decl_p, bool alias_p, spec_entry *elt,
       auto entry = ggc_alloc<spec_entry> ();
       *entry = *elt;
       *slot = entry;
-
-      if (alias_p)
-	{
-	  elt->spec = TREE_TYPE (elt->spec);
-	  gcc_checking_assert (elt->spec);
-	}
     }
-
-  if (!decl_p || alias_p)
+  else
     {
       auto *slot = type_specializations->find_slot_with_hash (elt, hash, INSERT);
 
diff --git a/gcc/testsuite/g++.dg/modules/pr99425-1_b.H b/gcc/testsuite/g++.dg/modules/pr99425-1_b.H
index 53d28b4ef5e..e75b3129dd3 100644
--- a/gcc/testsuite/g++.dg/modules/pr99425-1_b.H
+++ b/gcc/testsuite/g++.dg/modules/pr99425-1_b.H
@@ -15,5 +15,5 @@ inline void widget (Cont parm)
   ssize (parm);
 }
 
-// { dg-final { scan-lang-dump {Read:-[0-9]*'s alias spec merge key \(new\) type_decl:'::make_signed_t'\n  ...  Read:-[0-9]*'s type spec merge key \(new\) type_decl:'::make_signed'\n  Read:-[0-9]*'s named merge key \(matched\) template_decl:'::template ssize'} module } }
+// { dg-final { scan-lang-dump {Read:-[0-9]*'s decl spec merge key \(new\) type_decl:'::make_signed_t'\n  ...  Read:-[0-9]*'s type spec merge key \(new\) type_decl:'::make_signed'\n  Read:-[0-9]*'s named merge key \(matched\) template_decl:'::template ssize'} module } }
 
diff --git a/gcc/testsuite/g++.dg/modules/tpl-alias-1_a.H b/gcc/testsuite/g++.dg/modules/tpl-alias-1_a.H
index 37002ee9ae1..14a25be586f 100644
--- a/gcc/testsuite/g++.dg/modules/tpl-alias-1_a.H
+++ b/gcc/testsuite/g++.dg/modules/tpl-alias-1_a.H
@@ -6,4 +6,4 @@
 // { dg-final { scan-lang-dump {Writing decl tmpl spec:-[0-9]* template_decl:'::allocator_traits<::allocator<long int>>::template rebind_alloc<_Up>'} module } }
 // { dg-final { scan-lang-dump {Writing decl tmpl spec:-[0-9]* type_decl:'::allocator_traits<::allocator<long int>>::template rebind_alloc<_Up>'} module } }
 
-// { dg-final { scan-lang-dump {Writing:-[0-9]*'s alias spec merge key \(specialization\) type_decl:'::allocator_traits<::allocator<long int>>::rebind_alloc<long int>'} module } }
+// { dg-final { scan-lang-dump {Writing:-[0-9]*'s decl spec merge key \(specialization\) type_decl:'::allocator_traits<::allocator<long int>>::rebind_alloc<long int>'} module } }
diff --git a/gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H b/gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H
new file mode 100644
index 00000000000..76917f778e0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/tpl-alias-2_a.H
@@ -0,0 +1,15 @@
+// PR c++/103994
+// { dg-additional-options -fmodule-header }
+// { dg-module-cmi {} }
+
+template<class>
+struct A {
+  template<class> using key_arg = int;
+};
+
+struct B {
+  template<class T>
+  void f() {
+    using type = A<B>::key_arg<T>;
+  }
+};
diff --git a/gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C b/gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C
new file mode 100644
index 00000000000..44fa5f42757
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/tpl-alias-2_b.C
@@ -0,0 +1,9 @@
+// PR c++/103994
+// { dg-additional-options -fmodules-ts }
+
+import "tpl-alias-2_a.H";
+
+int main() {
+  B b;
+  b.f<int>();
+}
-- 
2.44.0.117.g43072b4ca1