New hack mode: require typehints for definitions in that file, but allow calling non-Hack code

[fredemmott]

Request from Davey on freenode.

Behavior should be:

• Every definition in that file requires a type hint; same as strict, unlike partial or decl
• Still allow calling untyped functions; like partial or decl, unlike strict

[jwatzman]

I can definitely see use cases for this; any objections I have are basically "we already have too many modes". @elgenie, @pikatchu, @gabelevi, any thoughts?

[dshafik]

I see this goal as a migratory mode: an indicator (and the type checker will complain if it's not) that the current file is ready to go strict, however it still calls into untyped functions as you haven't gotten around to updating them yet.

[lexidor]

I am going over old issues on this repository, to see which ones apply to the current versions of hhvm.

This matches the current behavior of // strict mode. I wasn't involved with hhvm in 2014, when this issue was made, and I have two conflicting ideas about what the issue author could have meant.
Either untyped means, a Hack function in a partial or decl file, with missing parameter / return types or it means an undeclared / php function.

PHP functions can not be called in hhvm anymore, since PHP support was dropped with the release of hhvm 4.0.0.
Strict code can call // partial and // decl code since at least hhvm 3.24.

[jwatzman]

Yeah I suspect "calling untyped functions" likely referred to the ancient assume_php, which used to be available in partial/decl but not strict. AFAIK it's entirely gone now and has been for a very long time.