c++: Fix instantiation of imported temploid friends [PR114275]

c++: Fix instantiation of imported temploid friends [PR114275]

This patch fixes a number of issues with the handling of temploid friend
declarations.

The primary issue is that instantiations of friend declarations should
attach the declaration to the same module as the befriending class, by
[module.unit] p7.1 and [temp.friend] p2; this could be a different
module from the current TU, and so needs special handling.

The other main issue here is that we can't assume that just because name
lookup didn't find a definition for a hidden class template, that it
doesn't exist at all: it could be a non-exported entity that we've
nevertheless streamed in from an imported module.  We need to ensure
that when instantiating template friend classes that we return the same
TEMPLATE_DECL that we got from our imports, otherwise we will get later
issues with 'duplicate_decls' (rightfully) complaining that they're
different when trying to merge.

This doesn't appear necessary for function templates due to the existing
name lookup handling already finding these hidden declarations.

	PR c++/105320
	PR c++/114275

gcc/cp/ChangeLog:

	* cp-tree.h (propagate_defining_module): Declare.
	(lookup_imported_hidden_friend): Declare.
	* decl.cc (duplicate_decls): Also check if hidden decls can be
	redeclared in this module.
	* module.cc (imported_temploid_friends): New.
	(init_modules): Initialize it.
	(trees_out::decl_value): Write it; don't consider imported
	temploid friends as attached to a module.
	(trees_in::decl_value): Read it.
	(get_originating_module_decl): Follow the owning decl for an
	imported temploid friend.
	(propagate_defining_module): New.
	* name-lookup.cc (get_mergeable_namespace_binding): New.
	(lookup_imported_hidden_friend): New.
	* pt.cc (tsubst_friend_function): Propagate defining module for
	new friend functions.
	(tsubst_friend_class): Lookup imported hidden friends.  Check
	for valid module attachment of existing names.  Propagate
	defining module for new classes.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-friend-10_a.C: New test.
	* g++.dg/modules/tpl-friend-10_b.C: New test.
	* g++.dg/modules/tpl-friend-10_c.C: New test.
	* g++.dg/modules/tpl-friend-10_d.C: New test.
	* g++.dg/modules/tpl-friend-11_a.C: New test.
	* g++.dg/modules/tpl-friend-11_b.C: New test.
	* g++.dg/modules/tpl-friend-12_a.C: New test.
	* g++.dg/modules/tpl-friend-12_b.C: New test.
	* g++.dg/modules/tpl-friend-12_c.C: New test.
	* g++.dg/modules/tpl-friend-12_d.C: New test.
	* g++.dg/modules/tpl-friend-12_e.C: New test.
	* g++.dg/modules/tpl-friend-12_f.C: New test.
	* g++.dg/modules/tpl-friend-13_a.C: New test.
	* g++.dg/modules/tpl-friend-13_b.C: New test.
	* g++.dg/modules/tpl-friend-13_c.C: New test.
	* g++.dg/modules/tpl-friend-13_d.C: New test.
	* g++.dg/modules/tpl-friend-13_e.C: New test.
	* g++.dg/modules/tpl-friend-13_f.C: New test.
	* g++.dg/modules/tpl-friend-13_g.C: New test.
	* g++.dg/modules/tpl-friend-14_a.C: New test.
	* g++.dg/modules/tpl-friend-14_b.C: New test.
	* g++.dg/modules/tpl-friend-14_c.C: New test.
	* g++.dg/modules/tpl-friend-14_d.C: New test.
	* g++.dg/modules/tpl-friend-9.C: New test.

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