Clang rejects constant expression in requires-clause as 'non-constant expression', which is a misleading diagnostic #81333
Description
Code
template <int __v> struct integral_constant {
static const int value = __v;
};
template <class _From, class _To>
constexpr bool is_convertible_v = __is_convertible(_From, _To);
template <class _Tp>
constexpr bool is_copy_constructible_v = __is_constructible(_Tp, _Tp);
template <class...> class tuple;
template <class T0> struct tuple<T0> {
template <class U0>
requires is_copy_constructible_v<U0>
tuple(U0);
};
struct ConvertibleFrom;
static_assert(is_convertible_v<tuple<int>, tuple<ConvertibleFrom>>);Command line
clang++ -std=c++20 file.cpp
Diagnostic
file.cpp:11:14: error: substitution into constraint expression resulted in a non-constant expression
11 | requires is_copy_constructible_v<U0>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
file.cpp:7:42: note: while checking constraint satisfaction for template 'tuple<int>' required here
7 | constexpr bool is_copy_constructible_v = __is_constructible(_Tp, _Tp);
| ^~~~~~~~~~~~~~~~~~
file.cpp:7:42: note: in instantiation of function template specialization 'tuple<int>::tuple<tuple<int>>' requested here
file.cpp:11:14: note: in instantiation of variable template specialization 'is_copy_constructible_v<tuple<int>>' requested here
11 | requires is_copy_constructible_v<U0>
| ^
file.cpp:11:14: note: while substituting template arguments into constraint expression here
11 | requires is_copy_constructible_v<U0>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
file.cpp:5:62: note: while checking constraint satisfaction for template 'tuple<ConvertibleFrom>' required here
5 | constexpr bool is_convertible_v = __is_convertible(_From, _To);
| ^
file.cpp:5:62: note: in instantiation of function template specialization 'tuple<ConvertibleFrom>::tuple<tuple<int>>' requested here
file.cpp:15:15: note: in instantiation of variable template specialization 'is_convertible_v<tuple<int>, tuple<ConvertibleFrom>>' requested here
15 | static_assert(is_convertible_v<tuple<int>, tuple<ConvertibleFrom>>);
| ^
file.cpp:11:14: note: initializer of 'is_copy_constructible_v<tuple<int>>' is unknown
11 | requires is_copy_constructible_v<U0>
| ^
file.cpp:7:16: note: declared here
7 | constexpr bool is_copy_constructible_v = __is_constructible(_Tp, _Tp);
| ^