Different overloads with std::function parameters is ambiguous with bind (sometimes)

Question

I have two overloads of a function foo which take different std::functions which results in an ambiguity issue for the latter when used with the re

Answer 1

The problem exists in how bind is allowed to be called. As cppreference states

If some of the arguments that are supplied in the call to g() are not matched by any placeholders stored in g, the unused arguments are evaluated and discarded.

In other words, you need to pass at least as many arguments as the underlying callable expects.

This means that the following is valid

int f();
auto b = std::bind(f);
b(1, 2, 3); // arguments aren't used

So saying

auto b = std::bind(ret_int)
b(1);

Works, with the 1 discarded, therefore the following is valid, and overload selection becomes ambiguous

std::function<void(int)> f = std::bind(ret_int);

The inverse is not true, however

std::function<int()> f = std::bind(take_int);

because take_int cannot be called with no arguments.

Takeaway: lambda > bind