Broken support for printing abstract classes with ostream adapter

[PiotrZSL]

ENV: GCC 10 + FMT 8.0.1 (master) + FMT_COMPILE + fmt::format_to + reference to abstract class as argument.
Abstract class got friend operator <<.

Result: Compilation fails.
Issue looks to be caused by std::numeric_limits.
Fails with: limits:317:7: error: invalid abstract return type ‘'.

Workaround:
format.h (ignored is_fast_float for classes)

 567 template<typename T, bool = std::is_class<T>::value>
 568 struct Workaround : std::false_type
 569 {
 570 };
 571
 572 template<typename T>
 573 struct Workaround<T, false> : bool_constant<std::numeric_limits<T>::is_iec559 && sizeof(T) <= sizeof(double)>
 574 {
 575
 576 };
 577
 578 template <typename T>
 579 using is_fast_float = Workaround<T>;

ostream.h (added & to std::declval to create reference to type instead of rvalue).

 75 template <typename T, typename Char> class is_streamable {
 76  private:
 77   template <typename U>
 78   static bool_constant<!std::is_same<decltype(std::declval<test_stream<Char>&>()
 79                                               << std::declval<U&>()),
 80                                      void_t<>>::value>

[vitaut]

Please provide a repro.

[PiotrZSL]

Example:

#include <fmt/format.h>
#include <fmt/compile.h>
#include <fmt/ostream.h>

struct Abstract
{
    virtual ~Abstract() = default;
    virtual void print() = 0;
    friend std::ostream& operator<<(std::ostream& out, Abstract&);
};


void testFail(Abstract& value)
{
    fmt::memory_buffer buf;
    fmt::format_to(std::back_inserter(buf), FMT_COMPILE("{}"), value);
}

Link: https://godbolt.org/z/KxzGEdhT7

GCC10 - Fails in ostream.h and format.h
GCC11 - Fails only in ostream.h due to lack of "const Abstract&" in "operator <<".

[vitaut]

Could you submit a PR by any chance?

[PiotrZSL]

Could you submit a PR by any chance?

Ok, I understand you mean pull request, probably I could...

[vitaut]

Fixed in ee0659f. With this change your example compiles (with a missing const added). Thanks for reporting.