[Bug c++/100779] New: Partial specialization with non-deducible template parameters accepted

[Bug c++/100779] New: Partial specialization with non-deducible template parameters accepted

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100779

            Bug ID: 100779
           Summary: Partial specialization with non-deducible template
                    parameters accepted
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

template <int> struct A;
template <class> struct B;
template <class T> struct B<A<sizeof(T)>> { };

This partial specialization should be rejected because its template parameter T
is not deducible.

[Bug c++/100779] Partial specialization with non-deducible template parameters accepted

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100779

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Another accepts-invalid example:

template <int> struct A;
template <class> struct B;
template <int N> struct B<A<N+0>> { };

[Bug c++/100779] Partial specialization with non-deducible template parameters accepted

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100779

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The problem is ultimately that for_each_template_parm doesn't honor
included_nondeduced_p correctly.

[Bug c++/100779] Partial specialization with non-deducible template parameters accepted

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100779

--- 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:9b94785dedb08b006419bec1a402614d9241317a

commit r12-1093-g9b94785dedb08b006419bec1a402614d9241317a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 27 12:36:39 2021 -0400

    c++: Relax rule for non-type arguments in partial specs [CWG1315]

    This implements the wording changes of CWG 1315, which permits non-type
    template arguments in a partial specialization to use template
    parameters more freely.  Delightfully, it seems the only change needed
    is to remove a single check from process_partial_specialization (amidst
    a bunch of whitespace changes).

    But that change alone revealed a latent problem with
for_each_template_parm:
    it ends up looking into some non-deduced contexts even when
    include_nondeduced_p is false.  This causes us to silently accept some
    partial specializations within the testsuite that contain non-deducible
    non-type template parameters (and that were previously rejected due to
    the rule that CWG 1315 relaxed).  For now this patch makes a minimal
    amount of changes to for_each_template_parm_r so that we continue to
    reject existing ill-formed partial specializations within the testsuite.
    I opened PR c++/100779 to track this issue.

            DR 1315
            PR c++/67593
            PR c++/96555

    gcc/cp/ChangeLog:

            * pt.c (process_partial_specialization): Don't error on a
            non-simple non-type template argument that involves template
            parameters.
            (for_each_template_parm_r): Don't walk TRAIT_EXPR, PLUS_EXPR,
            MULT_EXPR, or SCOPE_REF when include_nondeduced_p is false.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/partial16.C: New test.
            * g++.dg/template/partial17.C: New test.
            * g++.dg/template/partial18.C: New test.
            * g++.dg/template/partial19.C: New test.
            * g++.dg/cpp0x/pr68724.C: Adjust expected diagnostic for
            ill-formed partial specialization.
            * g++.dg/cpp0x/variadic38.C: Likewise.
            * g++.dg/cpp1z/pr81016.C: Likewise.
            * g++.dg/template/partial5.C: Likewise.
            * g++.old-deja/g++.pt/spec21.C: Likewise.