c++: comptypes ICE with BOUND_TEMPLATE_TEMPLATE_PARMs [PR107539]

c++: comptypes ICE with BOUND_TEMPLATE_TEMPLATE_PARMs [PR107539]

Here we end up giving the two BOUND_TEMPLATE_TEMPLATE_PARMs
C<decltype(f::t)> and C<decltype(g::t)> the same TYPE_CANONICAL because
the hash table that interns TYPE_CANONICAL for template type parameters
doesn't set the comparing_specializations flag which controls how
PARM_DECLs from different contexts compare equal.

Later, from spec_hasher::equal for the corresponding two specializations
A<C<decltype(f::t)>> and A<C<decltype(g::t)>>, we compare the two bound
ttps with comparing_specializations set hence they now (structurally)
compare different despite having the same TYPE_CANONICAL, and so we get
the error:

  internal compiler error: same canonical type node for different types
    'C<decltype (t)>' and 'C<decltype (t)>'

This suggests that we should be setting comparing_specializations from
ctp_hasher::equal to match spec_hasher::equal.  But doing so introduces
a separate ICE in cpp2a/concepts-placeholder3.C:

  internal compiler error: canonical types differ for identical types
    'auto [requires ::same_as<<placeholder>, decltype(f::x)>]' and
    'auto [requires ::same_as<<placeholder>, decltype(g::x)>]'

because norm_hasher::equal doesn't set comparing_specializations either.

I'm not sure when exactly we need to set comparing_specializations given
what it controls (TYPENAME_TYPE equality/hashing and PARM_DECL equality)
but it seems to be the conservative choice to set the flag wherever we
have a global hash table that relies on type equality.  To that end this
patch sets comparing_specializations in ctp_hasher and norm_hasher, as
well as in atom_hasher and sat_hasher for good measure.  This turns out
to be a compile time win of about 2% in some concepts tests, probably
because of the improved TYPENAME_TYPE hashing enabled by the flag.

	PR c++/107539

gcc/cp/ChangeLog:

	* constraint.cc (norm_hasher::hash, norm_hasher::equal): Set
	comparing_specializations.
	(sat_hasher::hash, sat_hasher::equal): Likewise.
	* cp-tree.h (atom_hasher::hash, atom_hasher::equal): Likewise.
	* pt.cc (ctp_hasher::hash, ctp_hasher::equal): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/template/canon-type-19.C: New test.