A missing include file is a constraint violation.

[Jonathan de Boyne Pollard]

J> It's implementation specific [...]

Only inasmuch as the implementation determines what "can be processed by the
implementation" means. However, a source file that _does not exist_ is going
to be a source file that cannot be "processed by the implementation" on
practically every implementation in existence. And thus a missing source file
is almost universally going to be a violation of an explicit constraint on
programs, and a diagnosable semantic error.