> A side effect of proposal #23367, which is needed for determining upper bound stack size, is guaranteed de-virtualization when there is only one Io implementation being used (also in debug builds!).

> In the less common case when a program instantiates more than one Io implementation, virtual calls done through the Io interface will not be de-virtualized, as that would imply doubling the amount of machine code generated, creating massive code bloat.

From the article

Having a limited number of known callees is already better than a virtual function (unrestricted function pointer). A compiler in theory could devirtualize every two-possible-callee callsite into `if (function_pointer == callee1) callee1() else callee2()` which then can be inlined at compile time or branch-predicted at runtime.

In any case, if you have two different implementations of something then you have to switch between them somewhere -- either at compile-time or link-time or load-time or run-time (or jit-time). The trick is to find an acceptable compromise of performance, (machine)code-bloat and API-simplicity.

> Wouldn't this only work if there's only one implementation throughout the entire compliation unit

in practice how often are people using more than one io in a program?