False exhaustivity for GADT under separate compilation

[eejbyfeldt]

Compiler version

3.7.2

Minimized code

In file1.scala

sealed trait N[T]
case class MakeTuple[T <: Tuple](v: T) extends N[T]

In file2.scala

object test {
  def t[T](expr: N[T]): Any =
    expr match {
      case MakeTuple(_) => ???
    }
}

Output

$ scalac file1.scala && scalac file2.scala
-- [E029] Pattern Match Exhaustivity Warning: file2.scala:3:4 ------------------
3 |    expr match {
  |    ^^^^
  |    match may not be exhaustive.
  |
  |    It would fail on pattern case: MakeTuple(_)
  |
  | longer explanation available when compiling with `-explain`
1 warning found

But compiling in a single scalac call e.g scalac file1.scala file2.scala gives no warnings.

Expectation

Ideally no warning should be produced. But consistently producing the warning would also be an improvement.

[eejbyfeldt]

3.3.5 produces the warning even when compiled together

$ scala-cli --scala-version 3.3.5 node.scala failure.scala
Warning: setting /home/ejbyfeldt/dev/scala_playground as the project root directory for this run.
Starting compilation server
Compiling project (Scala 3.3.5, JVM (17))
[warn] ./failure.scala:3:5
[warn] match may not be exhaustive.
[warn] 
[warn] It would fail on pattern case: MakeTuple(_)
[warn]     expr match {
[warn]     ^^^^
Compiled project (Scala 3.3.5, JVM (17))
[error]  No main class found

But that could just be that version does not properly support this case.

[eejbyfeldt]

Here is a similar possibly related case

$ cat codec.scala 
trait TC2[X]
sealed trait TC1[T]
case class Iterable[T, C <: scala.Iterable[T]](tag: TC1[C]) extends TC1[C]

$ cat using_codec.scala 
object test {
  def f[T](c: TC1[T]): Unit =
    c match {
      case Iterable(_) => ???
    }
}
$ scalac codec.scala && scalac using_codec.scala 
-- [E029] Pattern Match Exhaustivity Warning: using_codec.scala:3:4 ------------
3 |    c match {
  |    ^
  |    match may not be exhaustive.
  |
  |    It would fail on pattern case: Iterable(_)
  |
  | longer explanation available when compiling with `-explain`
1 warning found

[Gedochao]

Note: an easy way to reproduce this with Scala CLI is to force the 2 files into 2 separate compilation scopes.

rename file2.scala as file2.test.scala (to make it as part of the test scope for the reproduction) and compile like this:

scala-cli compile . --test --server=false -S 3.nightly
# -- [E029] Pattern Match Exhaustivity Warning: ~/23817/file2.test.scala:3:4                                                                                                                                           
# 3 |    expr match {
#   |    ^^^^
#   |    match may not be exhaustive.
#   |
#   |    It would fail on pattern case: MakeTuple(_)
#   |
#   | longer explanation available when compiling with `-explain`
1 warning found

This should simplify testing it with various versions.
Still occurs with 3.8.0-RC1-bin-20250915-afbb66b-NIGHTLY.