`<xutility>`: concept failed constraint has poor diagnostics.

For the following code:
https://godbolt.org/z/vYcqsW5qx

#include <algorithm>
#include <iterator>
#include <ranges>

struct it {
    it() noexcept;
    it &operator++() noexcept;
    it operator++(int) noexcept;
    int *operator*() const noexcept;
    int *operator->() const noexcept;
    bool operator==(const it &it) const noexcept;
    bool operator!=(const it &it) const noexcept;
};

template <typename T>
struct adapter {
    it begin() const;
    it end() const;
};

template <class T>
constexpr bool std::ranges::enable_borrowed_range<adapter<T>> = true;

using collection = adapter<int>;

bool pred(int *pdod) noexcept;

template <typename Col, typename Pred>
bool any_of_wrapper(Col &&col, Pred pred) {
    return std::any_of(col.begin(), col.end(), pred);
}

void cpp17() { std::begin(collection()); }
void cpp20() { std::ranges::begin(collection()); }

MSVC does not emit concepts diagnostics, so this bug report focuses on clang.

Using the Microsoft STL I see:

a.cpp(26,16): warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
void cpp17() { std::begin(collection()); }
               ^~~~~~~~~~ ~~~~~~~~~~~~
a.cpp(27,16): error: no matching function for call to object of type 'const _Begin::_Cpo'
void cpp20() { std::ranges::begin(collection()); }
               ^~~~~~~~~~~~~~~~~~
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.31.31103\include\xutility(2023,39): note: candidate template ignored: constraints not satisfied
      [with _Ty = adapter<int>]
            _NODISCARD constexpr auto operator()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                                      ^
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.31.31103\include\xutility(2022,27): note: because '_Choice<adapter<int> &>._Strategy != _St::_None'
      evaluated to false
                requires (_Choice<_Ty&>._Strategy != _St::_None)
                          ^
1 warning and 1 error generated.

The way the Microsoft STL is written the diagnostic is not useful. I have to look at the code and even then it does not tell me what the issue is.

If use the GCC STL I see:

<source>:34:16: error: no matching function for call to object of type 'const __cust_access::_Begin'
void cpp20() { std::ranges::begin(collection()); }
               ^~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/ranges_base.h:114:2: note: candidate template ignored: constraints not satisfied [with _Tp = adapter<int>]
        operator()[[nodiscard]](_Tp&& __t) const noexcept(_S_noexcept<_Tp&>())
        ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/ranges_base.h:111:11: note: because 'is_array_v<remove_reference_t<adapter<int> > >' evaluated to false
        requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
                 ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/ranges_base.h:111:50: note: and 'adapter<int>' does not satisfy '__member_begin'
        requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
                                                        ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/iterator_concepts.h:947:6: note: because 'decay_t<it>' (aka 'it') does not satisfy 'input_or_output_iterator':
          { __decay_copy(__t.begin()) } -> input_or_output_iterator;
            ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/iterator_concepts.h:616:5: note: because 'it' does not satisfy 'weakly_incrementable'
        && weakly_incrementable<_Iter>;
           ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/iterator_concepts.h:207:7: note: because 'typename iter_difference_t<_Iter>' would be invalid: no type named 'difference_type' in 'std::incrementable_traits<it>'
      using __iter_diff_t = typename
      ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/ranges_base.h:112:7: note: and 'adapter<int>' does not satisfy '__adl_begin'
          || __adl_begin<_Tp>
             ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.1/../../../../include/c++/12.0.1/bits/iterator_concepts.h:958:19: note: because '__decay_copy(begin(__t))' would be invalid: call to deleted function 'begin'
          { __decay_copy(begin(__t)) } -> input_or_output_iterator;
                         ^
1 error generated.

Which gives me a clear pointer of what failed.

Would it be possible to refactor the code in xutility related to ranges::begin constraints so that the compiler is able to emit proper constraints diagnostics?

The issue points to ranges::begin constraints in xutility; start there and compare the reported MSVC and GCC diagnostics. Refactor that area so a failing iterator constraint is surfaced directly in clang diagnostics, then verify the example reports the missing requirement rather than only a failed aggregate constraint.