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From: "richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99209] New: lambdas in function template signatures
 instantiated with wrong semantic context
Date: Mon, 22 Feb 2021 20:46:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99209

            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 =3D f_adl([]{})> void g(int =3D 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 semant=
ic
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 s=
uch
instantiated lambdas appear to be numbered after those lambdas for which the
ABI requires a numbering).=