Does syntax exist to call an unconstrained function that is hidden by a constrained function?

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I understand that with concepts, a constrained function (regardless how "loose" the constraint actually is) is always a better match than an unconstrained function. But is there any syntax to selectively call the unconstrained version of f() as in the sample code below? If not, would it be a good idea for compilers to warn about uncallable functions?

#include <iostream>

template <typename T> requires(true) 
void f() { std::cout << "Constrained\n"; }

template <typename T>
void f() { std::cout << "NOT Constrained\n"; }


int main() {
    f<int>();
}

https://godbolt.org/z/n164aTvd3

c++20 c++
1 Answers

Different overloads of a function are meant to all do the same thing. They may do it in different ways or on different kinds of objects, but at a conceptual level, all overloads of a function are supposed to do the same thing.

This includes constrained functions. By putting a constrained overload in a function overload set, you are declaring that this is a valid alternative method for doing what that overload set does. As such, if the constraint matches, then that's the function that should be called. Just like for parameters of different types in regular function overloading.

If you want to explicitly call an overload hidden by a constraint, you have already done something wrong in your design. Or more specifically, if some overload is completely hidden by one or more constrained overloads, you clearly have one more overload than you actually needed.

If the constraints match, the caller should be 100% fine with getting the constrained overload. And if this isn't the case, your design has a problem.

So no, there is no mechanism to do this. Just as there's no mechanism to bypass an explicitly specialized template and use the original version if your template arguments match the specialization.

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