Implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20

[daixtrose]

Compiling with g++ (Ubuntu 9.2.1-17ubuntu1~18.04.1) 9.2.1 20191102 I obtain warnings of the kind

fmt/include/fmt/format.h:1504:59: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]
 1504 |     out = write_int(out, num_digits, get_prefix(), specs, [=](iterator it) {

IMHO the solution is to explicitly specify this in the capture list, e.g.

out = write_int(out, num_digits, get_prefix(), specs, [=, this](iterator it) {

PR follows in a minute.