This is how C++ templates work. They are a form of polymorphism through how you use the object and not based on its type.

If you pass a type that cannot be used with the <= operator it will error in compile time.

Being able to do this is part of why C++ templates are much more powerful than Java/C# generics and why they enable a different (and alternative) form of polymorphism to inheritance and explicit interfaces.

I see. In e.g Swift, you'd have to define before hand that T1 could be used with <= while here I guess it adds in the method for "each" T1 that could be used with it.

I assume this just duplicates the method for each type at compile time instead of at runtime try to figure it out?

Damnit, now I wanna rewrite back to C++.

In C++ it just prints an error message and halts compilation if you try to use a type which doesn't support the semantics of the template function. It does not add functions which are not declared already.

That's what C++ concepts are for (maybe we'll see them in C++17).

It is all magic of Clang or GCC. Once you call the template with T1 the C++ compiler will enforce that your passed type supports the <= operator.