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// { dg-do run { target c++20 } }
template<class, class> constexpr bool is_same_v = false;
template<class T> constexpr bool is_same_v<T, T> = true;
template<class T, class U>
concept Same = is_same_v<T, U>;
template<class T, class U>
concept Diff = requires(T& t, U& u) { u - t; };
template<class I, class S>
int distance(I, S) { return 0; }
template<class I, Diff<I> S>
int distance(I first, S last)
{
return last - first;
}
template<class T>
struct I
{
template<class U>
requires Same<T, U>
friend int operator-(I const&, I<U> const&)
{
static_assert(Same<T, U>);
return 42;
}
};
int main()
{
return distance(I<int>{}, I<void>{});
}
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