Check return modes in type_argument after eliminating optionals

[stedolan]

There is a missing mode check in type_argument, in the case where it eliminates one or more optional arguments. With the following declarations in scope:

val mklocal : ?sep:string -> local_ (string -> string)
val use : (string -> string) -> unit

then the expression use mklocal currently compiles without error, ignoring the local_ return mode. This patch makes use mklocal an error, by checking that the return mode matches the expected mode.

I suspect that this has been unsound for a while, but it is difficult to make it segfault. (Since type_argument eta-expands, the obvious approach works without error). However, the bug causes the application inside type_argument's eta-expansion to have the wrong mode, which interacts badly with the mode-specific caml_curry and caml_apply functions.

The bug became much easier to trigger after ocaml-flambda/ocaml-jst#24, and showed up in real code, from which the testsuite example is extracted. I haven't managed to construct an example that segfaults before 24, but I think one probably exists.

[stedolan]

Closed in favour of the better fix in #735