[Bug c++/113563] New: Rejects capture of `this` in C++23 `this auto` lambda

[Bug c++/113563] New: Rejects capture of `this` in C++23 `this auto` lambda

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113563

            Bug ID: 113563
           Summary: Rejects capture of `this` in C++23 `this auto` lambda
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/KWv8n6zEG
struct S {
  int x_;
  void f() {
    [this](this auto) {
        return this;
    };
  }
};

GCC trunk complains:

<source>:5:16: error: invalid use of 'this' in non-member function
    5 |         return this;
      |                ^~~~

The error also happens with
- [this](this auto&&) { return this; }
- [this](this auto) { return x; }
- [this](this auto) { f(); }
- [&](this auto) { return this; }
- [&](this auto) { return x; }
- [&](this auto) { f(); }

My understanding is that all of these lambdas should be well-formed, just as
long as the lambda's call operator is eventually instantiated (if it's ever
instantiated at all) with a type that satisfies [expr.prim.lambda.closure]/5,
i.e. is either the closure type itself or a class type derived from the closure
type.


Btw, I do like that GCC eagerly and SFINAE-friendlily rejects `[&](this T) {}`.
I hope fixing this bug doesn't require undoing that feature.
(By "SFINAE-friendly" I mean https://godbolt.org/z/fK4f13343 )
See also
https://quuxplusone.github.io/blog/2024/01/23/capturing-lambda-deducing-this/

[Bug c++/113563] Rejects capture of `this` in C++23 `this auto` lambda

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113563

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #1 from Jiang An <de34 at live dot cn> ---
(In reply to Arthur O'Dwyer from comment #0)
> Btw, I do like that GCC eagerly and SFINAE-friendlily rejects `[&](this T)
> {}`. I hope fixing this bug doesn't require undoing that feature.
> (By "SFINAE-friendly" I mean https://godbolt.org/z/fK4f13343 )
> See also
> https://quuxplusone.github.io/blog/2024/01/23/capturing-lambda-deducing-this/

Hmm... after reading CWG2672
(https://cplusplus.github.io/CWG/issues/2672.html), I think the
SFINAE-friendliness is actually required since the parameter list of a lambda
expression is in the immediate context (only the lambda body is excluded).

[Bug c++/113563] Rejects capture of `this` in C++23 `this auto` lambda

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113563

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hewillk at gmail dot com

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
*** Bug 113810 has been marked as a duplicate of this bug. ***

[Bug c++/113563] Rejects capture of `this` in C++23 `this auto` lambda

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113563

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
It works if we capture 'this' indirectly, e.g. via an init-capture:

struct S {
  void f() {
    [this_=this](this auto) {
        return this_;
    };
  }
};

[Bug c++/113563] Rejects capture of `this` in C++23 `this auto` lambda

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113563

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rl.alt.accnt at gmail dot com

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
*** Bug 114257 has been marked as a duplicate of this bug. ***