A C type whose definition its header does not show

Decided

D-0040, settled by the language’s author: part of spec §18. Older text cites it as 18.40.

It imports as a handle: a type used behind ^T or ^mut T and never by value — no local, field, parameter, return or p^, and not copyable, because nothing outside the library knows its size. That is how RGFW_window, GLFWwindow, sqlite3 and FILE are handed out, and the functions taking one are then callable as they are in C. A type the header never mentions at all, declared in a system header it includes, is adopted the same way when every use of it is behind a pointer; where one is used by value the declaration that does so is left out instead, since its size really is unknown. Refusing such a header outright was the alternative, and would have shut out most C libraries (§15.2)