[Bug c++/61814] New: [c++1y] cannot use decltype on captured arg-pack

[Bug c++/61814] New: [c++1y] cannot use decltype on captured arg-pack

[tongari95 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61814

            Bug ID: 61814
           Summary: [c++1y] cannot use decltype on captured arg-pack
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tongari95 at gmail dot com

The code below has a lambda that captures the outer params t, for forwarding
matter, decltype on t has to be used.
```
    auto const pack = [](auto&&... t)
    {
        return [&](auto&& f)->decltype(auto)
        {
            return f(static_cast<decltype(t)>(t)...);
        };
    };

    int main(int argc, char** argv) {
        pack(1)([](int){});
        return 0;
    }
```
It works with clang 3.5, but g++ 4.9.0 complains:
error: 't' was not declared in this scope

The same also applies to `sizeof` or that kind of compile-time thing.

[Bug c++/61814] [c++1y] cannot use decltype on captured arg-pack

[oakad at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61814

Alexander Dubov <oakad at yahoo dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |oakad at yahoo dot com

--- Comment #1 from Alexander Dubov <oakad at yahoo dot com> ---
This problem still happens on 4.9.1.

For example, this example works:

int main(int argc, char **argv)
{
        int a{5};
        float z(0.5);
        [&a](float z_) -> void {
                decltype(a) b = a;
        }(z);
        return 0;
}

But this basic modification fails to compile (g++-4.9.1 -std=gnu++14):

int main(int argc, char **argv)
{
        int a{5};
        float z(0.5);
        [&a](auto z_) -> void {
                decltype(a) b = a;
        }(z);
        return 0;
}

[Bug c++/61814] [c++1y] cannot use decltype on captured arg-pack

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61814

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Alexander Dubov from comment #1)
> But this basic modification fails to compile (g++-4.9.1 -std=gnu++14):
> 
> int main(int argc, char **argv)
> {
>         int a{5};
>         float z(0.5);
>         [&a](auto z_) -> void {
>                 decltype(a) b = a;
>         }(z);
>         return 0;
> }

That seems to be a different issue, and is fixed on trunk.

[Bug c++/61814] [c++1y] cannot use decltype on captured arg-pack

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61814

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-10-23
                 CC|                            |jason at gcc dot gnu.org
      Known to work|                            |5.0
     Ever confirmed|0                           |1

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Actually the original problem is fixed on trunk too (but gives a different
error now), so maybe it is the same and can be reduced to:

int main()
{
  int a{5};
  [&a](auto z) -> void {
    decltype(a) b = a;
  }(1);
}

l.cc: In instantiation of ‘main()::<lambda(auto:1)> [with auto:1 = int]’:
l.cc:6:6:   required from here
l.cc:5:17: error: ‘a’ was not declared in this scope
     decltype(a) b = a;
                 ^

The error goes away if the lambda doesn't have a trailing-return-type, or if
it's not a generic lambda.

Jason, is it worth fixing this on the branch? It only affects C++14 mode.
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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/63628] New: [c++1y] cannot use decltype on captured variable in generic lambda
Date: Thu, 23 Oct 2014 12:41:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc628

            Bug ID: 63628
           Summary: [c++1y] cannot use decltype on captured variable in
                    generic lambda
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
                CC: daniel.kruegler at googlemail dot com, jason at gcc dot gnu.org,
                    oakad at yahoo dot com, tongari95 at gmail dot com
            Blocks: 54367

+++ This bug was initially created as a clone of Bug #61814 +++

The code below has a lambda that captures the outer params t, for forwarding
matter, decltype on t has to be used.
```
    auto const pack = [](auto&&... t)
    {
        return [&](auto&& f)->decltype(auto)
        {
            return f(static_cast<decltype(t)>(t)...);
        };
    };

    int main(int argc, char** argv) {
        pack(1)([](int){});
        return 0;
    }
```
It works with clang 3.5, but g++ 4.9.0 complains:
error: 't' was not declared in this scope

The same also applies to `sizeof` or that kind of compile-time thing.