Add support for 'std::variant' in C++17.
For C++17, if all the alternatives of a variant are formattable
the variant is now also formattable. In addition 'std::monostate'
is now formattable.
Moves implementation into 'std.h', and tests into 'std-test.cc'.
Avoid fold-expression since MSVC was crashing.
Add section for 'fmt/std.h' in API-docs.
Passing a function pointer to fmt::ptr results in:
In file included from /home/mac/git/fmt/test/gmock/gmock.h:238,
from /home/mac/git/fmt/test/format-test.cc:31:
.../fmt/test/format-test.cc: In member function ‘virtual void FormatterTest_FormatPointer_Test::TestBody()’:
.../fmt/test/format-test.cc:1486:56: error: no matching function for call to ‘ptr(void (&)(int, double, std::__cxx11::string))’
format("{}", fmt::ptr(function_pointer_test)));
with GCC and Clang. Let's add an overload to support that usage.
Unfortunately, MSVC would
consider the overload to be ambiguous for unknown reasons:
D:\a\fmt\fmt\test\format-test.cc(1485,1): error C2668: 'fmt::v7::ptr': ambiguous call to overloaded function [D:\a\fmt\build\test\format-test.vcxproj]
D:\a\fmt\fmt\include\fmt/format.h(3742,60): message : could be 'const void *fmt::v7::ptr<void,int,double,std::string>(T (__cdecl *)(int,double,std::string))' [D:\a\fmt\build\test\format-test.vcxproj]
with
[
T=void
]
D:\a\fmt\fmt\include\fmt/format.h(3735,42): message : or 'const void *fmt::v7::ptr<void(int,double,std::string)>(T (__cdecl *))' [D:\a\fmt\build\test\format-test.vcxproj]
with
[
T=void (int,double,std::string)
]
D:\a\fmt\fmt\test\format-test.cc(1486,1): message : while trying to match the argument list '(overloaded-function)' [D:\a\fmt\build\test\format-test.vcxproj]
but luckily this means that the overload is unnecessary in that case
anyway, so we can just make it conditional.
