g++-10 compilation error when including fmt/ostream.h

[MichaelTrikergiotis]

Including the fmt/ostream.h header when using g++-10 with -std=c++20, stops compilation with an error.

The issue doesn't exist with clang 12 or the latest MSVC.

compiler : g++ 10.3.0 with -std=c++20
OS : Ubuntu 21.04
fmt : 8.0.1

The issue also exists in fmt 8.0.
This issue doesn't exist in fmt 7.1.3.

Working example :

#define FMT_HEADER_ONLY 
#include "fmt/core.h"

int main()
{
	fmt::print("Works.\n");
}

The above compiled with g++-10 -std=c++20 with no problems.

Not working example, with link to godbolt : https://gcc.godbolt.org/z/7n6hPsdj8 :

#define FMT_HEADER_ONLY 
#include "fmt/core.h"
#include "fmt/ostream.h"

int main()
{
	fmt::print("Doesn't work.\n");
}

Trying to compile with g++-10 -std=c++20 :

In file included from bad.cpp:3:
fmt/ostream.h:123:8: error: partial specialization of ‘struct fmt::v8::detail::fallback_formatter<T, Char, typename std::enable_if<fmt::v8::detail::is_streamable<T, Char>::value, void>::type>’ after instantiation of ‘struct fmt::v8::detail::fallback_formatter<char [7], char, void>’ [-fpermissive]
  123 | struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fmt/ostream.h:123:8: error: partial specialization of ‘struct fmt::v8::detail::fallback_formatter<T, Char, typename std::enable_if<fmt::v8::detail::is_streamable<T, Char>::value, void>::type>’ after instantiation of ‘struct fmt::v8::detail::fallback_formatter<char [3], char, void>’ [-fpermissive]
fmt/ostream.h:123:8: error: partial specialization of ‘struct fmt::v8::detail::fallback_formatter<T, Char, typename std::enable_if<fmt::v8::detail::is_streamable<T, Char>::value, void>::type>’ after instantiation of ‘struct fmt::v8::detail::fallback_formatter<char [1], char, void>’ [-fpermissive]

[OnurKader]

Removing the header only macro seems to fix the issue (I hope header only is not mandatory).

godbolt

[MichaelTrikergiotis]

I need fmt in header only mode so removing the #define FMT_HEADER_ONLY macro is not possible for me.
The code in the post is just the smallest possible repro, and so I have included the macro on purpose.

[sunmy2019]

It is a design defect. To solve this, we need to reorganize the code.

#include "fmt/core.h"
#include "fmt/ostream.h"

// this file should be the last, but it is now included in the `core.h`
#include "fmt/format-inl.h"

[sunmy2019]

A temporary fix would be https://gcc.godbolt.org/z/xn3YenGxe

#include <fmt/core.h>
#include <fmt/ostream.h>
#define FMT_HEADER_ONLY
#undef FMT_FUNC
#define FMT_FUNC inline
#include <fmt/format-inl.h>

int main() {
  fmt::print("Doesn't work.\n");
}

[vitaut]

Fixed in 20931ba. Thanks for reporting!