[Bug c++/96363] bogus error with multiple constrained partial specialization forward declarations

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96363

--- Comment #4 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:97dc78d705a90c1ae83c78a7f2e24942cc3a6257

commit r13-779-g97dc78d705a90c1ae83c78a7f2e24942cc3a6257
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 26 09:43:14 2022 -0400

    c++: constrained partial spec forward decl [PR96363]

    Here during cp_parser_single_declaration for #2, we were calling
    associate_classtype_constraints for TPL<T> (the primary template type)
    before maybe_process_partial_specialization could get a chance to
    notice that we're in fact declaring a distinct constrained partial
    spec and not redeclaring the primary template.  This caused us to
    emit a bogus error about differing constraints b/t the primary template
    and #2's constraints.  This patch fixes this by moving the call to
    associate_classtype_constraints after the call to shadow_tag (which
    calls maybe_process_partial_specialization) and adjusting shadow_tag to
    use the return value of m_p_p_s.

    Moreover, if we later try to define a constrained partial specialization
    that's been declared earlier (as in the third testcase), then
    maybe_new_partial_specialization correctly notices it's a redeclaration
    and returns NULL_TREE.  But in this case we also need to update TYPE to
    point to the redeclared partial spec (it'll otherwise continue pointing
    to the primary template type, eventually leading to a bogus error).

            PR c++/96363

    gcc/cp/ChangeLog:

            * decl.cc (shadow_tag): Use the return value of
            maybe_process_partial_specialization.
            * parser.cc (cp_parser_single_declaration): Call shadow_tag
            before associate_classtype_constraints.
            * pt.cc (maybe_new_partial_specialization): Change return type
            to bool.  Take 'type' argument by mutable reference.  Set 'type'
            to point to the correct constrained specialization when
            appropriate.
            (maybe_process_partial_specialization): Adjust accordingly.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-partial-spec12.C: New test.
            * g++.dg/cpp2a/concepts-partial-spec12a.C: New test.
            * g++.dg/cpp2a/concepts-partial-spec13.C: New test.