Make implicit capture explicit for C++20

[daixtrose]

• Implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20, therefore this change.
• fixes Implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 #1668

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. There are some CI build failures: https://travis-ci.org/github/fmtlib/fmt/jobs/684199343

[daixtrose]

OK, it seems that mitigating the warning for a C++20 compiler is mutually exclusive to a C++14 compiler. Are you generally OK with the MACRO solution?

Looks like CI is passing

[vitaut]

Are you generally OK with the MACRO solution?

Sure but I suggest renaming FMT_CAPTURE_OF_THIS to FMT_CAPTURE_THIS.

[vitaut]

Another option is to change = to explicit capture - it might be slightly better than a macro.

[daixtrose]

Another option is to change = to explicit capture - it might be slightly better than a macro.

I am not sure how to do this such that it is portable since C++14 compilers reject any mention of this as error.

I suggest renaming FMT_CAPTURE_OF_THIS to FMT_CAPTURE_THIS.

ACK. Done.

[timsong-cpp]

[this, num_digits]. I think that's the only thing you need to capture in all cases?

[vitaut]

I am not sure how to do this such that it is portable since C++14 compilers reject any mention of this as error.

This seems to work (https://godbolt.org/z/jy-YYn):

struct S {
  void f() {
    int i;
    auto g = [i, this]() {};
    g();
  }
};

[daixtrose]

Ok, I will give it a try.

[daixtrose]

@vitaut explicit capture done. Now go fix your CI 🔂

[vitaut]

Thanks! Looks like a transient failure, I restarted the job.