Regression: "a match type could not be fully reduced" when flattening a list of tuples

[hegetim]

Compiler version

Affected: 3.5.0 and newer (tested on Scastie up to 3.6.1)
Latest working version: 3.4.3

Minimized code

case class MyNum(i: Int)

val l = List((MyNum(1), MyNum(1)))

l.flatMap(_.toList)

https://scastie.scala-lang.org/ZXMSKeMdT9SdHzvHdxVaQw

Output

Does not compile on 3.5.0 and newer:

    Found:    List[Tuple.Union[(_$1 : (Playground.MyNum, Playground.MyNum))]]
    Required: IterableOnce[Playground.MyNum]

    Note: a match type could not be fully reduced:

      trying to reduce  Tuple.Union[(_$1 : (Playground.MyNum, Playground.MyNum))]
      trying to reduce  Tuple.Fold[(_$1 : (Playground.MyNum, Playground.MyNum)), Nothing,
      [x, y] =>> x | y]
      failed since selector (_$1 : (Playground.MyNum, Playground.MyNum))
      does not uniquely determine parameters h, t in
        case h *: t => h | Tuple.Fold[t, Nothing, [x, y] =>> x | y]
      The computed bounds for the parameters are:
        h <: Playground.MyNum
        t <: Playground.MyNum *: EmptyTuple.type

Expectation

Should compile (as it does on 3.4.3) and give a flat list of MyNums.

[Gedochao]

Last good release: 3.5.0-RC1-bin-20240403-ece87c3-NIGHTLY
First bad release: 3.5.0-RC1-bin-20240404-a7f00e2-NIGHTLY
Bisect points at 0bf43b2

cc @sjrd

[sjrd]

Given the bisect commit, I suspect that the new error is legitimate. It fixed an unsoundness issue (#19746), so it may very well prevent compilation of code that worked more-or-less by chance before.

[hegetim]

Not sure if I get this right.

Is my usage of Tuple's toList function unsound? Or does the current implementation of toList just incorrectly rely on unsound behavior that has been fixed in 3.5.0?

The signature of toList is

inline def toList: List[Union[this.type]]

In its current state toList is quite limited. Any chance to get that improved?

[Gedochao]

@hegetim I think @sjrd meant that #19746 was an unsoundness issue (check the soundness label), and the fix for that caused this regression (that's where the bisect script points). We need to fix it separately.

[hegetim]

@Gedochao thanks for the explanation. So I guess this is not exactly a regression. The error is valid because in the declared return type, this.type is too narrow to be used in Tuple.Union. In order to improve the usability of Tuple's toList function, we probably have to make the widening of the tuple type explicit?

For example, changing toList into an extension method like this

extension [T <: Tuple](t: T)
  inline def toList: List[Tuple.Union[T]] = t.productIterator.toList.asInstanceOf[List[Tuple.Union[T]]]

would solve the issue (see https://scastie.scala-lang.org/42ADHhqzSEaaAmirzwxqyg)

I'm not sure if this can be done in a binary compatible way.

[Gedochao]

In order to improve the usability of Tuple's toList function, we probably have to make the widening of the tuple type explicit?

For example, changing toList into an extension method like this

extension [T <: Tuple](t: T)
inline def toList: List[Tuple.Union[T]] = t.productIterator.toList.asInstanceOf[List[Tuple.Union[T]]]
would solve the issue (see https://scastie.scala-lang.org/42ADHhqzSEaaAmirzwxqyg)

I'm not sure if this can be done in a binary compatible way.

@hegetim This seems out of scope here, potentially separate issue material.

Given the bisect commit, I suspect that the new error is legitimate. It fixed an unsoundness issue (#19746), so it may very well prevent compilation of code that worked more-or-less by chance before.

Alright, so it seems this behaviour is as expected.
Nothing to do here, closing this.