Inline match does not work on List

[jto]

The following example was tested in a worksheet, created from dotty-example-project (commit b135f5a) and Dotty 0.21.0-RC1

minimized code

inline def test(cs: List[Int]) <: Any =
  inline cs match
    case 1 :: Nil => 1
    case _ => scala.compiletime.error("Not one")

test(List(1)) // compilation error ("Not one")

expectation

in the given example, the first case should be matched and the value 1 of type 1 should be inlined.

[nicolasstucki]

It would require an inline parameter (see #8060). Also removing the default case helps to see the term that is not reduced.

class Test {
  inline def test(inline cs: List[Int]) <: Any =
    inline cs match
      case 1 :: Nil => 1

  test(1 :: Nil)
  test(List(1))
}

-- Error: i8103.scala:6:6 ------------------------------------------------------
6 |  test(1 :: Nil)
  |  ^^^^^^^^^^^^^^
  |  cannot reduce inline match with
  |   scrutinee:  {
  |    Nil.::[Int](1)
  |  } : List[Int]
  |   patterns :  case ::.unapply[Int](1, Nil):::[Int]

[nicolasstucki]

See https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/typer/Inliner.scala#L842

[jto]

Hi @nicolasstucki,
Thanks for your reply :)
Looking forward to test inline params!

[odersky]

Should we close this?

[jto]

I just tested on master and it seems to still fail :(

scala> class Test {
     |   inline def test(inline cs: List[Int]) <: Any =
     |     inline cs match
     |       case 1 :: Nil => 1
     |
     |   test(1 :: Nil)
     |   test(List(1))
     | }
6 |  test(1 :: Nil)
  |  ^^^^^^^^^^^^^^
  |  cannot reduce inline match with
  |   scrutinee:  {
  |    Nil.::[Int](1)
  |  } : List[Int]
  |   patterns :  case ::.unapply[Int](1, Nil):::[Int]
  | This location contains code that was inlined from rs$line$1:3

Am I missing something ?

[abgruszecki]

If I remember correctly, all inline pattern matching works based on scrutinee type, not the "shape" of the argument. Here the type of the scrutinee is List[Int], so a pattern for :: won't match.

[jto]

@AleksanderBG I don't understand what you mean by pattern matching works based on scrutinee type. Could you explain how would you rewrite the example ?

[abgruszecki]

@jto to make it work, you mean? The below code works:

scala> class Test {                                                                                                                                                                                                
     |   inline def test(inline cs: List[Int]) <: Any =
     |     inline cs match
     |       case 1 :: _ => 1
     | 
     |   val i: 1 = test(new ::[1](1, Nil))
     | }
// defined class Test

And so does the following:

scala> class Test {                                                                                                                                                                                                
     |   inline def test(inline cs: List[Int]) <: Any =
     |     inline cs match
     |       case _ :: _ => 1
     | 
     |   val i: 1 = test(new ::(1, Nil))
     | }
// defined class Test

But neither of the below:

scala> class Test {                                                                                                                                                                                                
     |   inline def test(inline cs: List[Int]) <: Any =
     |     inline cs match
     |       case _ :: Nil => 1
     | 
     |   val i: 1 = test(new ::(1, Nil))
     | }
6 |  val i: 1 = test(new ::(1, Nil))
  |             ^^^^^^^^^^^^^^^^^^^^
  |             cannot reduce inline match with
  |              scrutinee:  {
  |               new ::[Int](1, Nil)
  |             } : ::[Int]
  |              patterns :  case ::.unapply[Int](_, Nil):::[Int]
  | This location contains code that was inlined from rs$line$3:3

scala> class Test {                                                                                                                                                                                                
     |   inline def test(inline cs: List[Int]) <: Any =
     |     inline cs match
     |       case 1 :: _ => 1
     | 
     |   val i: 1 = test(new ::(1, Nil))
     | }
6 |  val i: 1 = test(new ::(1, Nil))
  |             ^^^^^^^^^^^^^^^^^^^^
  |             cannot reduce inline match with
  |              scrutinee:  {
  |               new ::[Int](1, Nil)
  |             } : ::[Int]
  |              patterns :  case ::.unapply[Int](1, _):::[Int]
  | This location contains code that was inlined from rs$line$3:3

It's clear to us that inline patmat semantics are a bit confusing, but it's just that no one got to improving this part yet.

[jto]

Okay I understand now. Thanks for the examples.

[abgruszecki]

Closing as a duplicate of #6781.