do fails to typecheck when using a non-generic alias

[lePerdu]

do fails to typecheck when it is type is specified using an alias which is "fully-applied" (i.e. all generics have been substituted with concrete types). Equivalent expressions using flat_map, not using the fully-applied alias, or even wrapping the expression in a nested let binding whose type is not specified all typecheck fine.

Example

type OptionInt = Option Int

let works_1 =
    do a = Some 1
    Some (a + 1)

let works_2 : Option Int =
    do a = Some 1
    Some (a + 1)

let works_3 : OptionInt =
    flat_map (\a -> Some (a + 1)) (Some 1)

let works_4 : OptionInt =
    let inner =
        do a = Some 1
        Some (a + 1)
    inner

// Error with the do expression here
let doesnt_work : OptionInt =
    do a = Some 1
    Some (a + 1)

Error

error: Expected the following types to be equal
Expected:
    (Int -> std.types.Option a) -> std.types.Option Int -> std.types.Option a
Found: (Int -> std.types.Option a) -> std.types.Option Int -> test.OptionInt
1 errors were found during unification:
Types do not match:
    Expected: std.types.Option a
    Found: test.OptionInt