c++/modules: Avoid rechecking initializers when streaming NTTPs [PR116382]

When reading an NTTP we call get_template_parm_object which delegates
setting of DECL_INITIAL to the general cp_finish_decl procedure, which
calls check_initializer to validate and record it.

Apart from being unnecessary (it must have already been validated by the
writing module), this also causes errors in cases like the linked PR, as
validating may end up needing to call lazy_load_pendings to determine
any specialisations that may exist which violates assumptions of the
modules streaming code.

This patch works around the issue by adding a flag to
get_template_parm_object to disable these checks when not needed.

	PR c++/116382

gcc/cp/ChangeLog:

	* cp-tree.h (get_template_parm_object): Add check_init param.
	* module.cc (trees_in::tree_node): Pass check_init=false when
	building NTTPs.
	* pt.cc (get_template_parm_object): Prevent cp_finish_decl from
	validating the initializer when check_init=false.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-nttp-1_a.C: New test.
	* g++.dg/modules/tpl-nttp-1_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

5 files changed, 35 insertions, 6 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 039c707..a9ce44b 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7626,7 +7626,8 @@ enum { nt_opaque = false, nt_transparent = true };
 extern tree alias_template_specialization_p     (const_tree, bool);
 extern tree dependent_alias_template_spec_p     (const_tree, bool);
 extern bool dependent_opaque_alias_p            (const_tree);
-extern tree get_template_parm_object		(tree expr, tree mangle);
+extern tree get_template_parm_object		(tree expr, tree mangle,
+						 bool check_init = true);
 extern tree tparm_object_argument		(tree);
 extern bool explicit_class_specialization_p     (tree);
 extern bool push_tinst_level                    (tree);
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index c3218bd..0a4ceff 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -9938,7 +9938,11 @@ trees_in::tree_node (bool is_use)
 	tree name = tree_node ();
 	if (!get_overrun ())
 	  {
-	    res = get_template_parm_object (init, name);
+	    /* We don't want to check the initializer as that may require
+	       name lookup, which could recursively start lazy loading.
+	       Instead we know that INIT is already valid so we can just
+	       apply that directly.  */
+	    res = get_template_parm_object (init, name, /*check_init=*/false);
 	    int tag = insert (res);
 	    dump (dumper::TREE)
 	      && dump ("Created nttp object:%d %N", tag, name);
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 32d164f..76edc7a 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7361,10 +7361,11 @@ create_template_parm_object (tree expr, tsubst_flags_t complain)
 static GTY(()) hash_map<tree, tree> *tparm_obj_values;
 
 /* Find or build an nttp object for (already-validated) EXPR with name
-   NAME.  */
+   NAME.  When CHECK_INIT is false we don't need to process the initialiser,
+   it's already been done.  */
 
 tree
-get_template_parm_object (tree expr, tree name)
+get_template_parm_object (tree expr, tree name, bool check_init/*=true*/)
 {
   tree decl = get_global_binding (name);
   if (decl)
@@ -7385,11 +7386,20 @@ get_template_parm_object (tree expr, tree name)
     {
       /* If EXPR contains any PTRMEM_CST, they will get clobbered by
 	 lower_var_init before we're done mangling.  So store the original
-	 value elsewhere.  */
-      tree copy = unshare_constructor (expr);
+	 value elsewhere.  We only need to unshare EXPR if it's not yet
+	 been processed.  */
+      tree copy = check_init ? unshare_constructor (expr) : expr;
       hash_map_safe_put<hm_ggc> (tparm_obj_values, decl, copy);
     }
 
+  if (!check_init)
+    {
+      /* The EXPR is the already processed initializer, set it on the NTTP
+	 object now so that cp_finish_decl doesn't do it again later.  */
+      DECL_INITIAL (decl) = expr;
+      DECL_INITIALIZED_P (decl) = 1;
+    }
+
   pushdecl_top_level_and_finish (decl, expr);
 
   return decl;
diff --git a/gcc/testsuite/g++.dg/modules/tpl-nttp-1_a.C b/gcc/testsuite/g++.dg/modules/tpl-nttp-1_a.C
new file mode 100644
index 0000000..46f6ecd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/tpl-nttp-1_a.C
@@ -0,0 +1,8 @@
+// PR c++/116382
+// { dg-additional-options "-fmodules-ts -std=c++20" }
+// { dg-module-cmi m:a }
+
+module m:a;
+template <typename> struct X {};
+template <X<int> nttp> struct index {};
+template struct index<{}>;
diff --git a/gcc/testsuite/g++.dg/modules/tpl-nttp-1_b.C b/gcc/testsuite/g++.dg/modules/tpl-nttp-1_b.C
new file mode 100644
index 0000000..a3241e7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/tpl-nttp-1_b.C
@@ -0,0 +1,6 @@
+// PR c++/116382
+// { dg-additional-options "-fmodules-ts -std=c++20" }
+// { dg-module-cmi m }
+
+export module m;
+import :a;