[Bug c++/105797] [11/12/13 Regression] Internal compiler error: Segmentation fault ( fn_type_unification -> satisfaction_cache::satisfaction_cache -> iterative_hash_template_arg )
cvs-commit at gcc dot gnu.org
Fri Jun 3 13:09:15 GMT 2022
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <firstname.lastname@example.org>:
Author: Patrick Palka <email@example.com>
Date: Fri Jun 3 09:08:41 2022 -0400
c++: find_template_parameters and PARM_DECLs [PR105797]
As explained in r11-4959-gde6f64f9556ae3, the atom cache assumes two
equivalent expressions (according to cp_tree_equal) must use the same
template parameters (according to find_template_parameters). This
assumption turned out to not hold for TARGET_EXPR, which was addressed
by that commit.
But this assumption apparently doesn't hold for PARM_DECL either:
find_template_parameters walks its DECL_CONTEXT but cp_tree_equal by
default doesn't consider DECL_CONTEXT unless comparing_specializations
is set. Thus in the first testcase below, the atomic constraints of #1
and #2 are equivalent according to cp_tree_equal, but according to
find_template_parameters the former uses T and the latter uses both T
and U (surprisingly).
We could fix this assumption violation by setting comparing_specializations
in the atom_hasher, which would make cp_tree_equal return false for the
two atoms, but that seems overly pessimistic here. Ideally the atoms
should continue being considered equivalent and we instead fix
find_template_paremeters to return just T for #2's atom.
To that end this patch makes for_each_template_parm_r stop walking the
DECL_CONTEXT of a PARM_DECL. This should be safe to do because
tsubst_copy / tsubst_decl only substitutes the TREE_TYPE of a PARM_DECL
and doesn't bother substituting the DECL_CONTEXT, thus the only relevant
template parameters are those used in its type. any_template_parm_r is
currently responsible for walking its TREE_TYPE, but I suppose it now makes
sense for for_each_template_parm_r to do so instead.
In passing this patch also makes for_each_template_parm_r stop walking
the DECL_CONTEXT of a VAR_/FUNCTION_DECL since doing so after walking
DECL_TI_ARGS is redundant, I think.
I experimented with not walking DECL_CONTEXT for CONST_DECL, but the
second testcase below demonstrates it's necessary to walk it.
* pt.cc (for_each_template_parm_r) <case FUNCTION_DECL, VAR_DECL>:
Don't walk DECL_CONTEXT.
<case PARM_DECL>: Likewise. Walk TREE_TYPE.
<case CONST_DECL>: Simplify.
(any_template_parm_r) <case PARM_DECL>: Don't walk TREE_TYPE.
* g++.dg/cpp2a/concepts-decltype4.C: New test.
* g++.dg/cpp2a/concepts-memfun3.C: New test.
More information about the Gcc-bugs