c++ modules: uninstantiated template friend class [PR104234]

Here we're not clearing DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P for
the instantiated/injected template friend class B, which confuses a
later call to get_originating_module_decl for B.  This patch fixes this
by clearing the flag in tsubst_friend_class (as is already done for
template friend functions by r11-5730-gf7aeb823d9b0de).

After fixing that, we still fail to compile the testcase, rejecting the
later definition of B with

  friend-6_a.C:10:26: error: cannot declare ‘struct B<T>’ in a different module

ultimately because DECL_MODULE_ATTACH_P wasn't set on the original
(injected) declaration of B.  This patch fixes this by calling
set_originating_module in tsubst_friend_class, but for that to work it
seems we need to relax the assert in this latter function since
get_originating_module_decl when called on the TYPE_DECL for B returns
the corresponding TEMPLATE_DECL.

(Alternatively we can instead call set_originating_module on the
TYPE_DECL B as soon as it's created in lookup_template_class (which is
what pushtag does), which doesn't need this assert change because at
this point the TYPE_DECL doesn't have any TEMPLATE_INFO so
get_originating_module_decl becomes a no-op.  Would that be preferable?)

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

	PR c++/104234

gcc/cp/ChangeLog:

	* module.cc (set_originating_module): Document default argument.
	Relax assert to look through DECL_TEMPLATE_RESULT in the result
	of get_originating_module_decl.
	* pt.cc (tsubst_friend_class): Clear
	DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P and call
	set_originating_module on the instantiated template friend class.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/friend-6_a.C: New test.

gcc/cp/module.cc

@@ -18843,14 +18843,18 @@ set_defining_module (tree decl)
 }
 
 void
-set_originating_module (tree decl, bool friend_p ATTRIBUTE_UNUSED)
+set_originating_module (tree decl, bool friend_p /* = false */)
 {
   set_instantiating_module (decl);
 
   if (!DECL_NAMESPACE_SCOPE_P (decl))
     return;
 
-  gcc_checking_assert (friend_p || decl == get_originating_module_decl (decl));
+  if (!friend_p)
+    {
+      tree o = get_originating_module_decl (decl);
+      gcc_checking_assert (STRIP_TEMPLATE (o) == decl);
+    }
 
   if (module_attach_p ())
     {

gcc/cp/pt.cc

@@ -11520,6 +11520,9 @@ tsubst_friend_class (tree friend_tmpl, tree args)
 	  CLASSTYPE_TI_ARGS (TREE_TYPE (tmpl))
 	    = INNERMOST_TEMPLATE_ARGS (CLASSTYPE_TI_ARGS (TREE_TYPE (tmpl)));
 
+	  DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P (tmpl) = false;
+	  set_originating_module (DECL_TEMPLATE_RESULT (tmpl));
+
 	  /* Substitute into and set the constraints on the new declaration.  */
 	  if (tree ci = get_constraints (friend_tmpl))
 	    {

gcc/testsuite/g++.dg/modules/friend-6_a.C

@@ -0,0 +1,10 @@
+// PR c++/104234
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi pr104234 }
+export module pr104234;
+
+template<class> struct A {
+  template<class T> friend struct B;
+};
+A<int> a;
+template<class T> struct B { };