Regression: Generic error "got T, expected T"

[svermeulen]

Hard to explain this one. I tried reducing it down as much as possible.

The following compiles fine for previous versions of teal. For current version (master) we get error main.tl:11:26: argument 1: argument 1: got T, expected T:

local record Mapper<T1>
   map:function<T2>(self, selector:(function(T1):T2))
end

local function to_mapper<T>(_values:{T}):Mapper<T>
   error("unimplemented")
end

local function _run<T>(values:{T})
   to_mapper(values):map(function(_:T):integer return 5 end)
end

_run({
   "five"
})

[svermeulen]

Can confirm that #906 fixes this