Add explicit this to lambda

[Chocobo1]

Implicit capture of this via [=] is deprecated in C++20 [-Wdeprecated]

[arvidn]

clang-tidy flags this at: https://github.com/arvidn/libtorrent/actions/runs/5951657979/job/16141913252?pr=7492#step:7:100
as

/home/runner/work/libtorrent/libtorrent/src/session_impl.cpp:5895:10: error: explicit capture of 'this' with a capture default of '=' is a C++20 extension [clang-diagnostic-c++20-extensions,-warnings-as-errors]
                        , [=, this](aux::listen_socket_handle const& sock
                              ^

Is explicit capture of this not supported before C++20? If so, it seems a bit drastic to require C++20 support, rather than suppressing the deprecation warning

[Chocobo1]

Is explicit capture of this not supported before C++20?

Sadly this seems to be the case. From https://en.cppreference.com/w/cpp/language/lambda#Lambda_capture:

struct S2 { void f(int i); };
void S2::f(int i)
{
   [=, this] {};  // until C++20: Error: this when = is the default
                  // since C++20: OK, same as [=]
}

Although clang doesn't produce a hard error, there is still a warning: https://godbolt.org/z/o8Eh91Wx4

Now I've updated the PR to include a conditional to ensure compatibility with all cpp versions.

[arvidn]

I think it might be a mistake that this is captured in some of those cases

[arvidn]

@Chocobo1 does this work for you? #7502

[Chocobo1]

Superseded by #7502.