please add support to fmt::printf for passing volatile arguments

[janwilmans]

https://godbolt.org/z/R2rR5j

#define FMT_HEADER_ONLY
#include <fmt/printf.h>

struct foo
{
    enum fooEnum { test };
    fooEnum i;
};

int main()
{
    volatile foo f; 
    
    printf("%d", f.i);
    std::string s = fmt::sprintf("%d", f.i); // 
}

gives:

/opt/compiler-explorer/libs/fmt/6.1.0/include/fmt/core.h: In instantiation of 'fmt::v6::internal::value<Context>::value(const T&) [with T = volatile foo::fooEnum; Context = fmt::v6::basic_printf_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>]':

/opt/compiler-explorer/libs/fmt/6.1.0/include/fmt/core.h:1122:39:   required from 'fmt::v6::internal::value<Context> fmt::v6::internal::make_arg(const T&) [with bool IS_PACKED = true; Context = fmt::v6::basic_printf_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>; T = volatile foo::fooEnum; typename std::enable_if<IS_PACKED, int>::type <anonymous> = 0]'

/opt/compiler-explorer/libs/fmt/6.1.0/include/fmt/core.h:1212:58:   required from 'fmt::v6::format_arg_store<Context, Args>::format_arg_store(const Args& ...) [with Context = fmt::v6::basic_printf_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>; Args = {volatile foo::fooEnum}]'

/opt/compiler-explorer/libs/fmt/6.1.0/include/fmt/core.h:1226:18:   required from 'fmt::v6::format_arg_store<Context, Args ...> fmt::v6::make_format_args(const Args& ...) [with Context = fmt::v6::basic_printf_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>; Args = {volatile foo::fooEnum}]'

/opt/compiler-explorer/libs/fmt/6.1.0/include/fmt/printf.h:618:77:   required from 'std::basic_string<Char> fmt::v6::sprintf(const S&, const Args& ...) [with S = char [3]; Args = {volatile foo::fooEnum}; Char = char]'

<source>:15:43:   required from here

/opt/compiler-explorer/libs/fmt/6.1.0/include/fmt/core.h:798:18: error: invalid conversion from 'const volatile void*' to 'const void*' [-fpermissive]

     custom.value = &val;

[vitaut]

Should be fixed in f72a905. Thanks for reporting.

[janwilmans]

This also fixed: https://godbolt.org/z/GW7G_9

enum class Foo
{
    somevalue
};

int main()
{
    Foo f{};

    printf("%d", f);
    std::string s = fmt::sprintf("%d", f);
}