Code doesn't typecheck when moving a type alias between modules

Start with the linked minimal repository and reproduce the failure using the two referenced commits, running spago build and comparing the module placement of the type alias. Use src/Main.purs and the compiler error to isolate the typechecking path. Done means the alias can move between modules while both versions compile without the Pos instance error.

Description

I discovered a weird compiler error in a large project, and narrowed it down to an alias having to be defined in the right module. Moving the alias to another module breaks type checking, with the error being something about Pos not being implemented. See the below output:

compiler-bug-example on 󰘬 master via <=> v0.15.10 via  impure (nix-shell-env) took 2m43s
$ spago build
[1 of 1] Compiling Main
[1/1 NoInstanceFound] src/Main.purs:16:7

  16  bar = discreteFromDense
            ^^^^^^^^^^^^^^^^^

  No type class instance was found for

    Data.Typelevel.Num.Sets.Pos t0

  The instance head contains unknown type variables. Consider adding a type annotation.

  while checking that type forall (c :: Symbol) (u :: Symbol) (m :: Type) (n :: Type). Pos m => Pos n => UnitScale @Type c u (Tuple m n) => Approximation -> Dense c -> Tuple ... ...
    is at least as general as type Approximation -> Dense "SEK" -> Tuple (Discrete "SEK" "öre") (Dense "SEK")
  while checking that expression discreteFromDense
    has type Approximation -> Dense "SEK" -> Tuple (Discrete "SEK" "öre") (Dense "SEK")
  in value declaration bar

  where t0 is an unknown type

           Src   Lib   All
Warnings   0     0     0
Errors     1     0     1
[error] Failed to build.

To Reproduce

Steps to reproduce the behavior.

I have set up a minimal repository here

Steps:

• Check out this commit https://github.com/NomisIV/purescript-compiler-bug/commit/d558e589a6a29ed639987302910bf2d52a770fa7
• Code doesn't compile
• Check out this commit https://github.com/NomisIV/purescript-compiler-bug/commit/302ec3be754ab986fa89b899a79c8a0eb03572b8
• Code compiles

Expected behavior

Expected to be able to move type alias between modules.

Additional context

Add any other context about the problem here.

PureScript version

0.15.10