Implement Concepts v2

[lackhole]

Implement <concepts_v2>

• Support human-readable error message
• Compatible with old type_traits-like method

---

template<typename T, typename U, preview_require(
    (preview::integral<T> || preview::floating_point<T>) && preview::signed_integral<U>
)>
void foo(T, U) {}

foo("Hello, C++", 20);

gcc 11

error: no type named 'valid' in 'struct constraints_not_satisfied<integral<const char*>, at<0, 3>, because<constraints_not_satisfied<integral<const char*>, at<0, 1>, because<std::is_integral<const char*>, is_false> > >, and_, floating_point<const char*>, at<1, 3>, because<constraints_not_satisfied<floating_point<const char*>, at<0, 1>, because<std::is_floating_point<const char*>, is_false> > > >'

/preview/test/concepts_v2.cc:97:6: error: no matching function for call to 'foo(const char [11], int)'
   97 |   foo("hello, C++", 20);
      |   ~~~^~~~~~~~~~~~~~~~~~
/preview/test/concepts_v2.cc:33:6: note: candidate: 'template<class T, class U, typename decltype (preview::resolve_require(((integral<T> || floating_point<T>) && (! signed_integral<U>))))::valid <anonymous> > void foo(T, U)'
   33 | void foo(T, U) {}
      |      ^~~
/preview/test/concepts_v2.cc:33:6: note:   template argument deduction/substitution failed:
In file included from /preview/include/preview/concepts_v2.h:12,
from /preview/test/concepts_v2.cc:4:
/preview/include/preview/__concepts_v2/require.h:53:96: error: no type named 'valid' in 'struct constraints_not_satisfied<integral<const char*>, at<0, 3>, because<constraints_not_satisfied<integral<const char*>, at<0, 1>, because<std::is_integral<const char*>, is_false> > >, and_, floating_point<const char*>, at<1, 3>, because<constraints_not_satisfied<floating_point<const char*>, at<0, 1>, because<std::is_floating_point<const char*>, is_false> > > >'
   53 | #define preview_require(...) typename decltype(preview::resolve_require(__VA_ARGS__))::valid = true
      |                                                                                                ^~~~
/preview/test/concepts_v2.cc:30:34: note: in expansion of macro 'preview_require'
   30 | template<typename T, typename U, preview_require(
      |                                  ^~~~~~~~~~~~~~~

Clang

no type named 'valid' in 'constraints_not_satisfied<integral<const char *>, at<0, 3>, because<constraints_not_satisfied<integral<const char *>, at<0, 1>, because<std::is_integral<const char *>, is_false>>>, and_, floating_point<const char *>, at<1, 3>, because<constraints_not_satisfied<floating_point<const char *>, at<0, 1>, because<std::is_floating_point<const char *>, is_false>>>>'

/preview/test/concepts_v2.cc:97:3: error: no matching function for call to 'foo'
  foo("hello, C++", 20);
  ^~~
/preview/test/concepts_v2.cc:33:6: note: candidate template ignored: substitution failure [with T = const char *, U = int]: no type named 'valid' in 'constraints_not_satisfied<integral<const char *>, at<0, 3>, because<constraints_not_satisfied<integral<const char *>, at<0, 1>, because<std::is_integral<const char *>, is_false>>>, and_, floating_point<const char *>, at<1, 3>, because<constraints_not_satisfied<floating_point<const char *>, at<0, 1>, because<std::is_floating_point<const char *>, is_false>>>>'
void foo(T, U) {}
     ^