Opaque type parameter partially loses opacity when compiled separately

[gaelrenoux]

Compiler version

3.5.2

Minimized code

src/main/scala/definition.scala

opaque type SlotId = Int
opaque type TopicId = Int

opaque type SmallIdSet[I] = Long
extension [I](s: SmallIdSet[I]) {
  inline def has(id: I): Boolean = ???
}

src/test/scala/usage.scala

val id: TopicId = ???

val smallIdSet: SmallIdSet[SlotId] = ???
smallIdSet.has(id)

Output

The code compiles.

Expectation

The code shouldn't compile, as the expected argument for has is of type SlotId.

Further investigations

If both files are compiled together (both in src/main/scala or both in src/test/scala), the code doesn't compile, as expected.

The following line doesn't compile in usage.scala, as expected:

val otherSmallIdSet: SmallIdSet[TopicId] = smallIdSet

If defining the opaque type using a parameterized type in definition.scala, for example:

opaque type SmallIdSet[I] = Set[I]

the final line of usage.scala doesn't compile, as expected.

[som-snytt]

The symptom requires "separate compilation", not just "different compilation units" (meaning source files).

➜  snips scalac -d ~/sandbox i21946a.scala
➜  snips scalac -d ~/sandbox -cp ~/sandbox i21946b.scala
➜  snips scalac -d ~/sandbox i21946*.scala
-- [E007] Type Mismatch Error: i21946b.scala:5:23 ------------------------------
5 |def f = smallIdSet.has(id)
  |                       ^^
  |                       Found:    (id : TopicId)
  |                       Required: SlotId

[gaelrenoux]

Fixed the title and description, sorry for the vocabulary confusion.