[Bug c++/99209] New: lambdas in function template signatures instantiated with wrong semantic context

[Bug c++/99209] New: lambdas in function template signatures instantiated with wrong semantic context

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99209

            Bug ID: 99209
           Summary: lambdas in function template signatures instantiated
                    with wrong semantic context
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

Testcase:

constexpr decltype(auto) f_adl(auto a) { return f(a); }

namespace A {
    constexpr int f(auto) { return 0; }
    template<int = f_adl([]{})> void g(int = f_adl([]{})) {
        f_adl([]{});
    }
}

int main() {
    A::g();
}

Here, the f_adl calls for the default template argument and default function
argument incorrectly result in errors, because f can't be found by ADL.

I suspect this is because the lambdas are being created in the wrong semantic
context. (That is also visible in the output of __PRETTY_FUNCTION__ /
std::source_location::current().function_name() in the lambda and in the
mangling of the lambda closure type, but I think both of those are valid
implementation choices -- the function name is implementation-defined and such
instantiated lambdas appear to be numbered after those lambdas for which the
ABI requires a numbering).

[Bug c++/99209] lambdas in function template signatures instantiated with wrong semantic context

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99209

--- Comment #1 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
Here's a more interesting example: https://godbolt.org/z/83c36q


#include <iostream>

constexpr char f(...) { return 'g'; }
constexpr decltype(auto) f_adl(auto a) { return f(a); }

namespace A {
    constexpr char f(auto) { return 'A'; }
    template<char TemplateParam = f_adl([]{})> void g(char FunctionParam =
f_adl([]{})) {
        char Local = f_adl([]{});
        std::cout << TemplateParam << FunctionParam << Local;
    }
}

namespace B {
    constexpr char f(auto) { return 'B'; }
    void call() { A::g(); }
}

int main() { B::call(); }


This prints 'BgA', but should print 'AAA'. So the three lambdas actually
exhibit three different behaviors, not two.

[Bug c++/99209] lambdas in function template signatures instantiated with wrong semantic context

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99209

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2021-03-03
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c++/99209] lambdas in function template signatures instantiated with wrong semantic context

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99209

--- 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:f7280b04714c24af2854cf020b6e7b6ad3b7fba8

commit r13-2542-gf7280b04714c24af2854cf020b6e7b6ad3b7fba8
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Sep 8 10:49:04 2022 -0400

    c++: Add testcase for already fixed PR [PR99209]

    This was incidentally fixed by r13-806-g221acd67ca50f8.

            PR c++/99209

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/lambda-uneval17.C: New test.

[Bug c++/99209] lambdas in function template signatures instantiated with wrong semantic context

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99209

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=105652
   Target Milestone|---                         |12.2
         Resolution|---                         |FIXED
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
This appears to be fixed for GCC 12.2/trunk by the fix for PR105652.