c++/modules: Fix recursive dependencies [PR116317]

In cases like the linked PR we sometimes get mutually recursive
dependencies that both rely on the other to have been streamed as part
of their merge key information.  In the linked PR, this causes an ICE.

The root cause is that 'sort_cluster' is not correctly ordering the
dependencies; both the element_t specialisation and the
reverse_adaptor::first function decl depend on each other, but by
streaming element_t first it ends up trying to stream itself recursively
as part of calculating its own merge key, which apart from the checking
ICE will also cause issues on stream-in, as the merge key will not
properly stream.

There is a comment already in 'sort_cluster' describing this issue, but
it says:

     Finding the single cluster entry dep is very tricky and
     expensive.  Let's just not do that.  It's harmless in this case
     anyway.

However in this case it was not harmless: it's just somewhat luck that
the sorting happened to work for the existing cases in the testsuite.

This patch solves the issue by noting any declarations that rely on deps
first seen within their own merge key.  This declaration gets marked as
an "entry" dep; any of these deps that end up recursively referring back
to that entry dep as part of their own merge key do not.

Then within sort_cluster we can ensure that the entry dep is written to
be streamed first of its cluster; this will ensure that any other deps
are just emitted as back-references, and the mergeable dep itself will
structurally decompose.

	PR c++/116317

gcc/cp/ChangeLog:

	* module.cc
	(depset::DB_MAYBE_RECURSIVE_BIT): New flag.
	(depset::DB_ENTRY_BIT): New flag.
	(depset::is_maybe_recursive): New accessor.
	(depset::is_entry): New accessor.
	(depset::hash::writing_merge_key): New field.
	(trees_out::decl_value): Inform dep_hash while we're writing the
	merge key information for a decl.
	(depset::hash::add_dependency): Find recursive deps and mark the
	entry point.
	(sort_cluster): Ensure that the entry dep is streamed first.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/late-ret-4_a.H: New test.
	* g++.dg/modules/late-ret-4_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

0 files changed, 0 insertions, 0 deletions