constraints on eq

[arturbac]

feature request to put constraints on eq as this fixes unreadable complains for types being compared and streamed to ostream that do not have overloaded operator << for std::ostream & and fixes more important thing, error points to user code instead to ut internals.
For example when I added below concepts:

my_test_ut.cc:125:12: error: no matching function for call to 'eq'
  125 |     expect(eq(*it, strong_typed::v1));
      |            ^~
boost/ut.hpp:3075:30: note: candidate template ignored: constraints not satisfied [with TLhs = strong_typed, TRhs = strong_typed]
[..]
note: because 'strong_typed' does not satisfy 'stream_insertable'

orignal error output is a horror

include/boost/ut.hpp:1351:10: error: invalid operands to binary expression ('std::ostringstream' (aka 'basic_ostringstream<char>') and 'strong_typed')
 1351 |     out_ << detail::get(t);
 
 .... 300 lines of UT internals and without reference to user code

something like c++20

template<typename T>
concept stream_insertable = requires(T t, std::ostream& os) {
    { os << detail::get(t) } -> std::same_as<std::ostream&>;
};

template <detail::stream_insertable TLhs, detail::stream_insertable TRhs>
[[nodiscard]] constexpr auto eq(const TLhs& lhs, const TRhs& rhs) 

or c++17
template<typename T, typename = void>
struct is_stream_insertable : std::false_type {};

template<typename T>
struct is_stream_insertable<T,
    std::void_t<decltype(std::declval<std::ostream&>() << detail::get(std::declval<T>()))>
> : std::true_type {};

 
template <typename TLhs, typename TRhs, 
          typename = std::enable_if_t<is_stream_insertable<TLhs>::value && is_stream_insertable<TRhs>::value>>

[kris-jusiak]

Totally, thanks @arturbac . Happy to get it in as soon as possible if you are planning to create MR and have specific tests/uses cases in mind which you wanna cover. If not will follow-up with the fix. thanks.

[arturbac]

1. The same applies to gt, neq, lt, le, approx ..

Some minor requests just to have nice without warnings output.
2. I would like kindly ask for changing constant name

**boost/ut.hpp:108:8: warning: identifier '_unique_name_for_auto_detect_prefix_and_suffix_lenght_0123456789_struct'
 is reserved because it starts with '_' at global scope [-Wreserved-identifier]**

3. I would like kindly ask for fix for llvm libc++
   boost/ut.hpp:2324:5: warning: '__cpp_lib_format' is not defined, evaluates to 0 [-Wundef]

#if defined(__cpp_lib_format) or \
    (defined(_LIBCPP_VERSION) and _LIBCPP_VERSION >= 170000)
#define BOOST_UT_HAS_FORMAT
#endif

as this BOOST_UT_HAS_FORMAT is defined while __cpp_lib_format is not available

#if defined(BOOST_UT_HAS_FORMAT)
#if __cpp_lib_format >= 202207L

I would add deduction guide to classes like eq_or explicitly put typenames as template params

boost/ut.hpp:3064:10: warning: 'eq_' may not intend to support class template argument deduction [-Wctad-maybe-unsupported]

template <stream_insertable TLhs, stream_insertable TRhs>
[[nodiscard]] constexpr auto eq(const TLhs& lhs, const TRhs& rhs) {
  return detail::eq_<TLhs,TRhs>{lhs, rhs};