Swift Whole Module Optimization
For the None or Fast optimization levels the compiler is working with a single source file at a time. When the definition of the generic function is in the same source file as the calling code the compiler knows we are calling the function with integers and optimizes away unnecessary object handling code. This optimization known as Generic Specialization will only work when the compiler has visibility of the function definition meaning that the external definition runs more slowly.
It makes compilation slower, though.
