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From: "enricomaria.dean6elis at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/106957] New: compiler error when calling lambda in decltype
 in trailing return type of function templated on T when passing T{} to the
 lambda
Date: Fri, 16 Sep 2022 16:30:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106957

            Bug ID: 106957
           Summary: compiler error when calling lambda in decltype in
                    trailing return type of function templated on T when
                    passing T{} to the lambda
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: enricomaria.dean6elis at gmail dot com
  Target Milestone: ---

Created attachment 53586
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D53586&action=3Dedit
the output/error upon compilation

Repro on godbolt (https://godbolt.org/z/9T5aEnzYa)

Question on stackoverlow with a comment sayint that the code would be illeg=
al
anyway:
https://stackoverflow.com/questions/73747517/sfinae-ing-on-a-nullary-lambda=
-expression-calling-a-non-templated-function-on-a

Here's the code
```
#include <type_traits>

constexpr auto f =3D [](int){};

template<typename T>
auto canCallFOn(T) -> decltype([]{ f(T{}); }(), std::true_type{});

constexpr std::false_type canCallFOn(...) {
    return {};
}

int main() {
    static_assert(!canCallFOn(1));
    static_assert(!canCallFOn(""));
}
```=