[Bug c++/99885] New: CTAD fails for auto const& NTTP

[Bug c++/99885] New: CTAD fails for auto const& NTTP

[ldalessandro at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

            Bug ID: 99885
           Summary: CTAD fails for auto const& NTTP
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldalessandro at gmail dot com
  Target Milestone: ---

Following (much-reduced) code causes a CTAD failure in gcc but not clang
(https://godbolt.org/z/zWhWGj7PE) with both C++17 and C++20.

template <auto const& A>
struct Foo {};

template <auto const& A>
struct Bar {
    constexpr auto foo() const -> Foo<A> {
        return {};
    }
};

constexpr int a = 1;
constexpr Bar<a> bar;
Foo foo = bar.foo(); // <-- CTAD failure

[Bug c++/99885] CTAD fails for auto const& NTTP

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2021-04-13
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, this never worked.  Reduced:

template <int& V> struct A {};
int n = 1;
A a = A<n>{};

Looking into it.

[Bug c++/99885] CTAD fails for auto const& NTTP

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

--- Comment #2 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:2ccc05a5141506fde0e20dec702c717fd67bf6ee

commit r11-8175-g2ccc05a5141506fde0e20dec702c717fd67bf6ee
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Apr 14 08:54:30 2021 -0400

    c++: Fix deduction with reference NTTP [PR83476]

    In the testcase ref11.C below, during deduction for the call f(a),
    uses_deducible_template_parms returns false for the dependent
    specialization A<V> because the generic template argument V here is
    wrapped in an implicit INDIRECT_REF (formed from template_parm_to_arg).
    Since uses_deducible_template_parms returns false, unify_one_argument
    exits early without ever attempting to deduce 'n' for 'V'.  This patch
    fixes this by making deducible_expression look through such implicit
    INDIRECT_REFs.

    gcc/cp/ChangeLog:

            PR c++/83476
            PR c++/99885
            * pt.c (deducible_expression): Look through implicit
            INDIRECT_REFs as well.

    gcc/testsuite/ChangeLog:

            PR c++/83476
            PR c++/99885
            * g++.dg/cpp1z/class-deduction85.C: New test.
            * g++.dg/template/ref11.C: New test.

[Bug c++/99885] CTAD fails for auto const& NTTP

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the bug report, this is now fixed for GCC 11.  The underlying bug is
7 regression from the 4.9 era and is being tracked at PR83476.

[Bug c++/99885] CTAD fails for auto const& NTTP

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.0

[Bug c++/99885] CTAD fails for auto const& NTTP

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:904cbf6ae2a1e45b3a272365cca19f9dd9a27fa2

commit r10-9733-g904cbf6ae2a1e45b3a272365cca19f9dd9a27fa2
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 20 12:06:24 2021 -0400

    c++: Fix deduction with reference NTTP [PR83476]

    In the testcase ref11.C below, during deduction for the call f(a),
    uses_deducible_template_parms returns false for the dependent
    specialization A<V> because the generic template argument V here is
    wrapped in an implicit INDIRECT_REF (formed from template_parm_to_arg).
    Since uses_deducible_template_parms returns false, unify_one_argument
    exits early without ever attempting to deduce 'n' for 'V'.  This patch
    fixes this by making deducible_expression look through such implicit
    INDIRECT_REFs.

    gcc/cp/ChangeLog:

            PR c++/83476
            PR c++/99885
            * pt.c (deducible_expression): Look through implicit
            INDIRECT_REFs as well.

    gcc/testsuite/ChangeLog:

            PR c++/83476
            PR c++/99885
            * g++.dg/cpp1z/class-deduction85.C: New test.
            * g++.dg/template/ref11.C: New test.

    (cherry picked from commit 2ccc05a5141506fde0e20dec702c717fd67bf6ee)

[Bug c++/99885] CTAD fails for auto const& NTTP

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:c6432e17960799509c48df6b500a8d069110980b

commit r9-10154-gc6432e17960799509c48df6b500a8d069110980b
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Apr 14 08:54:30 2021 -0400

    c++: Fix deduction with reference NTTP [PR83476]

    In the testcase ref11.C below, during deduction for the call f(a),
    uses_deducible_template_parms returns false for the dependent
    specialization A<V> because the generic template argument V here is
    wrapped in an implicit INDIRECT_REF (formed from template_parm_to_arg).
    Since uses_deducible_template_parms returns false, unify_one_argument
    exits early without ever attempting to deduce 'n' for 'V'.  This patch
    fixes this by making deducible_expression look through such implicit
    INDIRECT_REFs.

    gcc/cp/ChangeLog:

            PR c++/83476
            PR c++/99885
            * pt.c (deducible_expression): Look through implicit
            INDIRECT_REFs as well.

    gcc/testsuite/ChangeLog:

            PR c++/83476
            PR c++/99885
            * g++.dg/cpp1z/class-deduction85.C: New test.
            * g++.dg/template/ref11.C: New test.

    (cherry picked from commit 2ccc05a5141506fde0e20dec702c717fd67bf6ee)

[Bug c++/99885] CTAD fails for auto const& NTTP

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885
Bug 99885 depends on bug 83476, which changed state.

Bug 83476 Summary: [9 Regression] Template argument deduction fails with reference template parameter
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83476

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

[Bug c++/99885] CTAD fails for auto const& NTTP

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99885

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.0                        |9.5