[Bug c++/94632] [10 Regression] canonical types differ for identical types since r10-7622-g12f55e030ed068d5

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94632

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:f83adb68ed9cef1fbd4c16447eb4e89676df9f62

commit r10-7787-gf83adb68ed9cef1fbd4c16447eb4e89676df9f62
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Apr 18 06:22:21 2020 -0400

    c++: spec_hasher::equal and PARM_DECLs [PR94632]

    In the testcase below, during specialization of c<int>::d, we build two
    identical specializations of the parameter type b<decltype(e)::k> -- one
when
    substituting into c<int>::d's TYPE_ARG_TYPES and another when substituting
into
    c<int>::d's DECL_ARGUMENTS.

    We don't reuse the first specialization the second time around as a
consequence
    of the fix for PR c++/56247 which made PARM_DECLs always compare different
from
    one another during spec_hasher::equal.  As a result, when looking up
existing
    specializations of 'b', spec_hasher::equal considers the template argument
    decltype(e')::k to be different from decltype(e'')::k, where e' and e'' are
the
    result of two calls to tsubst_copy on the PARM_DECL e.

    Since the two specializations are considered different due to the mentioned
fix,
    their TYPE_CANONICAL points to themselves even though they are otherwise
    identical types, and this triggers an ICE in
maybe_rebuild_function_decl_type
    when comparing the TYPE_ARG_TYPES of c<int>::d to its DECL_ARGUMENTS.

    This patch fixes this issue at the spec_hasher::equal level by ignoring the
    'comparing_specializations' flag in cp_tree_equal whenever the
DECL_CONTEXTs of
    the two parameters are identical.  This seems to be a sufficient condition
to be
    able to correctly compare PARM_DECLs structurally.  (This also subsumes the
    CONSTRAINT_VAR_P check since constraint variables all have empty, and
therefore
    identical, DECL_CONTEXTs.)

    gcc/cp/ChangeLog:

            PR c++/94632
            * tree.c (cp_tree_equal) <case PARM_DECL>: Ignore
            comparing_specializations if the parameters' contexts are
identical.

    gcc/testsuite/ChangeLog:

            PR c++/94632
            * g++.dg/template/canon-type-14.C: New test.