[Bug c++/110006] [11/12/13/14 Regression] friend function template with constraint doesn't match existing declaration

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110006

--- Comment #4 from GCC 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:686b5eb9c9ee623a604dde5c49fa11c23f384c62

commit r14-8770-g686b5eb9c9ee623a604dde5c49fa11c23f384c62
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 2 19:07:08 2024 -0500

    c++: requires-exprs and partial constraint subst [PR110006]

    In r11-3261-gb28b621ac67bee we made tsubst_requires_expr never partially
    substitute into a requires-expression so as to avoid checking its
    requirements out of order during e.g. generic lambda regeneration.

    These PRs however illustrate that we still sometimes do need to
    partially substitute into a requires-expression, in particular when it
    appears in associated constraints that we're directly substituting for
    sake of declaration matching or dguide constraint rewriting.  In these
    cases we're being called from tsubst_constraint during which
    processing_constraint_expression_p is true, so this patch checks this
    predicate to control whether we defer substitution or partially
    substitute.

    In turn, we now need to propagate semantic tsubst flags through
    tsubst_requires_expr rather than just using tf_none, notably for sake of
    dguide constraint rewriting which sets tf_dguide.

            PR c++/110006
            PR c++/112769

    gcc/cp/ChangeLog:

            * constraint.cc (subst_info::quiet): Accomodate non-diagnostic
            tsubst flags.
            (tsubst_valid_expression_requirement): Likewise.
            (tsubst_simple_requirement): Return a substituted _REQ node when
            processing_template_decl.
            (tsubst_type_requirement_1): Accomodate non-diagnostic tsubst
            flags.
            (tsubst_type_requirement): Return a substituted _REQ node when
            processing_template_decl.
            (tsubst_compound_requirement): Likewise.  Accomodate non-diagnostic
            tsubst flags.
            (tsubst_nested_requirement): Likewise.
            (tsubst_requires_expr): Don't defer partial substitution when
            processing_constraint_expression_p is true, in which case return
            a substituted REQUIRES_EXPR.
            * pt.cc (tsubst_expr) <case REQUIRES_EXPR>: Accomodate
            non-diagnostic tsubst flags.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/class-deduction-alias18.C: New test.
            * g++.dg/cpp2a/concepts-friend16.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>