Apparently, C++ concepts are only shallow.

#include <concepts>

template<class T>
T add(T a, T b)
{ return a + b;}

template<class T>
concept addable = requires(T a, T b)
{
    { add(a, b) } -> std::same_as<T>;
};

struct foo{};

static_assert(addable<int>);
static_assert(addable<foo>);  // Passes?!

int main()
{
    add(foo{}, foo{});  // Will not compile
}

Would it be possible somehow make evaluation of addable instantiate the template, so foo no longer appears addable? It is possible to introduce a constraint on the add template, but I am looking for a more general approach:

1. The compiler finds that the only match is a template
2. The compiler tries to instantiate the template
3. Instantiation fails somewhere down the expansion tree, and the concept evaluates to false