Match Types for Type Projection not working in 3.4.0-RC1

[chungonn]

The minimized code below compiles in Scala 3.3.1 but is failing in Scala 3.4.0-RC1. Is this a bug or an expected behaviour in Scala 3.4.0? If yes, is there any workaround for this?

trait Foo:
  case class Bar[A](value: A)

object FooBar extends Foo

type UnwrapTypes[Xs] =
  Xs match
    case EmptyTuple => Xs
    case x *: xs => UnwrapTypes[x] *: UnwrapTypes[xs]
    case Foo#Bar[x] => UnwrapTypes[x]
    case String => String
    case Int => Int

val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]]
val tup: (Int, String)  = ??? : UnwrapTypes[(FooBar.Bar[Int], FooBar.Bar[String])]

Output

[info] compiling 1 Scala source to /home/chungonn/development/experiments/scala3-types-match/target/scala-3.4.0-RC1/classes ...
[error] -- [E007] Type Mismatch Error: /home/chungonn/development/experiments/scala3-types-match/src/main/scala/matchtypes/Main.scala:16:13 
[error] 16 |val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]]
[error]    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |Found:    matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[Int]]
[error]    |Required: Int
[error]    |
[error]    |Note: a match type could not be fully reduced:
[error]    |
[error]    |  trying to reduce  matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[Int]]
[error]    |  failed since selector matchtypes.FooBar.Bar[Int]
[error]    |  does not match  case matchtypes.Foo#Bar[x] => matchtypes.UnwrapTypes[x]
[error]    |  and cannot be shown to be disjoint from it either.
[error]    |  Therefore, reduction cannot advance to the remaining cases
[error]    |
[error]    |    case String => String
[error]    |    case Int => Int
[error]    |
[error]    | longer explanation available when compiling with `-explain`
[error] -- [E007] Type Mismatch Error: /home/chungonn/development/experiments/scala3-types-match/src/main/scala/matchtypes/Main.scala:17:26 
[error] 17 |val tup: (Int, String)  = ??? : UnwrapTypes[(FooBar.Bar[Int], FooBar.Bar[String])]
[error]    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |Found:    (matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[Int]],
[error]    |  matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[String]])
[error]    |Required: (Int, String)
[error]    |
[error]    |Note: a match type could not be fully reduced:
[error]    |
[error]    |  trying to reduce  matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[Int]]
[error]    |  failed since selector matchtypes.FooBar.Bar[Int]
[error]    |  does not match  case matchtypes.Foo#Bar[x] => matchtypes.UnwrapTypes[x]
[error]    |  and cannot be shown to be disjoint from it either.
[error]    |  Therefore, reduction cannot advance to the remaining cases
[error]    |
[error]    |    case String => String
[error]    |    case Int => Int
[error]    |
[error]    | longer explanation available when compiling with `-explain`
[error] two errors found
[error] (Compile / compileIncremental) Compilation failed

[soronpo]

@sjrd Maybe you can respond.

[jchyb]

Minimized (tuples does not seem to cause any problems here):

trait Foo:
  case class Bar[A](value: A)

object FooBar extends Foo

type UnwrapTypes[Xs] =
  Xs match
    case Foo#Bar[x] => UnwrapTypes[x]
    case Int => Int

val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]]

[chungonn]

@jchyb your code does not compile both in my environment (Ubuntu 23.10, Java 17, Scala 3.4.0-RC1) and in scastie

https://scastie.scala-lang.org/IaEVOKQWSHqmoTmZH7U7zg

Here's the compilation error with your code. Also, I reported the problem using type matching the Tuple type :)

[error] -- [E007] Type Mismatch Error: /home/chungonn/development/experiments/scala3-types-match/src/main/scala/matchtypes/Main2.scala:14:13 
[error] 14 |val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]]
[error]    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |Found:    matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[Int]]
[error]    |Required: Int
[error]    |
[error]    |Note: a match type could not be fully reduced:
[error]    |
[error]    |  trying to reduce  matchtypes.UnwrapTypes[matchtypes.FooBar.Bar[Int]]
[error]    |  failed since selector matchtypes.FooBar.Bar[Int]
[error]    |  does not match  case matchtypes.Foo#Bar[x] => matchtypes.UnwrapTypes[x]
[error]    |  and cannot be shown to be disjoint from it either.
[error]    |  Therefore, reduction cannot advance to the remaining case
[error]    |
[error]    |    case Int => Int
[error]    |
[error]    | longer explanation available when compiling with `-explain`
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 0 s, completed Jan 16, 2024, 6:24:27 AM

Here's my original reported code in scastie

https://scastie.scala-lang.org/21Lz8vxoQMOIwwCO4KySQw

Both above snippet code will only compile using 3.3.1

[sjrd]

Looks like a bug, indeed.