How to check that a function result is constexpr even when the argument is not?

constexpr functions can yield constexpr results even when their arguments are not constexpr, if they are unused. Such is, for example, std::integral_constant::operator T:

#include <utility>std::integral_constant<int, 42> a;constexpr int b = a; // Compiles!

How can I make a concept to test this property of a function?

For example, I want the following to work: run on gcc.godbolt.org

#include <utility>struct A {explicit constexpr operator bool() const {return true;}};struct B {int x = 1; explicit constexpr operator bool() const {return x;}};template <typename T>concept C = requires(T t){std::bool_constant<t>{};};static_assert(C<A>);static_assert(!C<B>);

But this gives me:

<source>:7:46: error: constraint variable 't' cannot be used in an evaluated context    7 | concept C = requires(T t){std::bool_constant<t>};      |                                              ^