[Bug c++/102670] New: Erroneous "missing template arguments" message for wrapper of ADL function template

[Bug c++/102670] New: Erroneous "missing template arguments" message for wrapper of ADL function template

[friedkeenan at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102670

            Bug ID: 102670
           Summary: Erroneous "missing template arguments" message for
                    wrapper of ADL function template
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: friedkeenan at protonmail dot com
  Target Milestone: ---

With the following code, GCC outputs an erroneous error message of "missing
template arguments":

#include <utility>

namespace ns {

    struct S { };

    template<int I>
    constexpr int adl(const S &) {
        return I;
    }

}

namespace redirect {

    template<typename T, int I>
    concept can_call_adl = requires(T &&t) {
        adl<I>(std::forward<T>(t));
    };

    template<int I>
    struct adl_fn {
        template<can_call_adl<I> T>
        constexpr decltype(auto) operator ()(T &&t) const {
            return adl<I>(std::forward<T>(t));
        }
    };

    namespace {

        template<int I>
        constexpr inline adl_fn<I> adl{};

    }

}

static_assert(redirect::can_call_adl<ns::S, 3>);

int main() {
    // return adl<3>(ns::S{});
    return redirect::adl<3>(ns::S{});
}

Godbolt link: https://godbolt.org/z/or5n8EM6q

As you can see, even though ns::S satisfies the redirect::can_call_adl concept,
when the templated call operator is instantiated, it thinks the code is
invalid. If you remove the indirection, no error message is presented and
everything works as expected. Additionally, Clang handles this code just fine.

If you however use the following code, everything works fine:

#include <utility>

namespace ns {

    struct S { };

    template<int I>
    constexpr int adl(const S &) {
        return I;
    }

}

namespace redirect {

    namespace _adl {

        /* Poison pill. */
        template<int I>
        void adl() = delete;

        template<typename T, int I>
        concept can_call_adl = requires(T &&t) {
            adl<I>(std::forward<T>(t));
        };

        template<int I>
        struct adl_fn {
            template<can_call_adl<I> T>
            constexpr decltype(auto) operator ()(T &&t) const {
                return adl<I>(std::forward<T>(t));
            }
        };

    }

    namespace {

        template<int I>
        constexpr inline _adl::adl_fn<I> adl{};

    }

}

static_assert(redirect::_adl::can_call_adl<ns::S, 3>);

int main() {
    // return adl<3>(ns::S{});
    return redirect::adl<3>(ns::S{});
}

Godbolt link: https://godbolt.org/z/jocaKrjbW

This sort of functionality is desirable for making a wrapper that handles both
ADL-discovered `get` functions and `get` member functions (as structured
binding allows both), similar to the std::ranges::begin etc. wrappers.

[Bug c++/102670] Erroneous "missing template arguments" message for wrapper of ADL function template

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102670

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-11-03

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.

[Bug c++/102670] Erroneous "missing template arguments" message for wrapper of ADL function template

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102670

--- 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:6fa8e0896c6ec96eddcedb2b92502a7bbb525c03

commit r12-5372-g6fa8e0896c6ec96eddcedb2b92502a7bbb525c03
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Nov 18 10:04:27 2021 -0500

    c++: unqual lookup performed twice w/ template-id ADL [PR102670]

    Here we're incorrectly performing unqualified lookup of 'adl' again at
    substitution time for the call adl<I>(t) (for which name lookup at parse
    time found nothing) which causes us to reject the testcase because the
    second unqualified lookup finds the later-declared variable template
    'adl', leading to confusion.  Fixed thusly.

    The testcase concepts-recursive-sat1.C needed to be adjusted to use ADL
    proper instead of relying on this incorrect second unqualified lookup.

            PR c++/102670

    gcc/cp/ChangeLog:

            * pt.c (tsubst_copy_and_build) <case CALL_EXPR>: When looking
            for an identifier callee in the koenig_p case, also look through
            TEMPLATE_ID_EXPR.  Use tsubst_copy to substitute through the
            template arguments of the template-id.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-recursive-sat1.C: Adjust to use ADL
            proper.
            * g++.dg/cpp2a/fn-template23.C: New test.

[Bug c++/102670] Erroneous "missing template arguments" message for wrapper of ADL function template

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102670

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12, thanks for the bug report.