c++/modules: Detect exposures of TU-local entities

c++/modules: Detect exposures of TU-local entities

Currently, the modules streaming code implements some checks for
declarations in the CMI that reference (some kinds of) internal-linkage
entities, and errors if so.  This patch expands on that support to
implement the logic for exposures of TU-local entities as defined in
[basic.link] since P1815.

This will cause some code that previously errored in modules to start
compiling; for instance, template specialisations of internal linkage
functions.

However, some code that previously appeared valid will with this patch
no longer compile, notably some kinds of usages of internal linkage
functions included from the GMF.  This appears to be related to P2808
and FR-025, however as yet there doesn't appear to be consensus for
changing these rules so I've implemented them as-is.

This patch leaves a couple of things out.  In particular, a couple of
the rules for what is a TU-local entity currently seem to me to be
redundant; I've left them as FIXMEs to be handled once I can find
testcases that aren't adequately supported by the other logic here.

Additionally, there are some exceptions for when naming a TU-local
entity is not always an exposure; I've left support for this to a
follow-up patch for easier review, as it has broader implications for
streaming.

TU-local lambdas are also not yet properly implemented, due to other
bugs with regards to LAMBDA_TYPE_EXTRA_SCOPE not being set in all cases
that it probably should be (see also PR c++/116568).  We can revisit
this once that issue has been fixed.

Finally, this patch makes a couple of small adjustments to the modules
streaming logic to prune any leftover TU-local deps (that aren't
erroneous exposures).  This is required for this patch to ensure that
later stages don't get confused by any leftover TU-local entities
floating around.

gcc/cp/ChangeLog:

	* tree.cc (decl_linkage): Treat DECL_SELF_REFERENCE_P like
	DECL_IMPLICIT_TYPEDEF_P.
	* name-lookup.cc (do_namespace_alias): Fix linkage.
	* module.cc (DB_IS_INTERNAL_BIT): Rename to...
	(DB_TU_LOCAL_BIT): ...this.
	(DB_REFS_INTERNAL_BIT): Rename to...
	(DB_EXPOSURE_BIT): ...this.
	(depset::hash::is_internal): Rename to...
	(depset::hash::is_tu_local): ...this.
	(depset::hash::refs_internal): Rename to...
	(depset::hash::is_exposure): ...this.
	(depset::hash::is_tu_local_entity): New function.
	(depset::hash::has_tu_local_tmpl_arg): New function.
	(depset::hash::is_tu_local_value): New function.
	(depset::hash::make_dependency): Check for TU-local entities.
	(depset::hash::add_dependency): Make current an exposure
	whenever it references a TU-local entity.
	(depset::hash::add_binding_entity): Don't create bindings for
	any TU-local entity.
	(depset::hash::finalize_dependencies): Rename flags and adjust
	diagnostic messages to report exposures of TU-local entities.
	(depset::tarjan::connect): Don't include any TU-local depsets.
	(depset::hash::connect): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/block-decl-2.C: Adjust messages.
	* g++.dg/modules/internal-1.C: Adjust messages, remove XFAILs.
	* g++.dg/modules/linkage-2.C: Adjust messages, remove XFAILS.
	* g++.dg/modules/internal-3.C: New test.
	* g++.dg/modules/internal-4_a.H: New test.
	* g++.dg/modules/internal-4_b.C: New test.

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