Module imports with different capitalization create phantom type conflicts

[jiribenes]

This is a fun bug that seems pretty hard to describe, but basically, there are two different "code paths" that reach a "different" Person even though it's actually the same exact file and type (which is a fact that Effekt doesn't understand).

Take a look at the minimal reproduction:

common.effekt

module common

record Person(name: String, birthYear: Int)

personutils.effekt

module personutils

import Common
// !!! notice the capitalisation here:
//  if you make it lowercase, everything works again

def calculateAge(p: Person, currentYear: Int) =
  p.birthYear - currentYear

main.effekt

module main

import common
import personutils

def main() = {
  val jolene = Person("Jolene", 1953)
  val age = calculateAge(jolene, 2025)
  // ^ ERROR: Expected Person but got Person
  println(age)
}

[jiribenes]

Notes:

• if you import a module of the exact same name multiple times, everything is OK
• if you import both common and Common, the code now returns:

Ambiguous overload.
There are multiple overloads, which all would type check:
- common::Person: (String, Int) => Person
- common::Person: (String, Int) => Person

[jiribenes]

One minute debugging:

Tracing findSource leads to:

findSource: Trying to find: common
... ~> Some(FileSource(./common.effekt,UTF-8))
findSource: Trying to find: personutils
... ~> Some(FileSource(./personutils.effekt,UTF-8))
findSource: Trying to find: Common
... ~> Some(FileSource(./Common.effekt,UTF-8))

Now if you try getting the content of the Common file source, you get the same contents as the ./common.effekt file has.
The Source type and its .content comes from Kiama, and .content just uses scala.io.Source.fromFile, so that's where the investigation would lead next.
https://github.com/effekt-lang/kiama/blob/ca71c77bf9b04d1f6721660322631cfad7a2def5/shared/src/main/scala/kiama/util/Source.scala#L112

[jiribenes]

Had one more idea and one more minute. If you:

1. remove the module common from common.effekt
2. in main.effekt import both common and Common

then the error is:

There are multiple overloads, which all would type check:

• common::Person: (String, Int) => Person
• Common::Person: (String, Int) => Person

But if you revert step 1 (keep module common in common), the error is:

There are multiple overloads, which all would type check:

• common::Person: (String, Int) => Person
• common::Person: (String, Int) => Person

This seems to suggest that the module xyz declaration is at least partially taken into account for this...