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3.2.0 regression around match types

See original GitHub issue

Compiler version

3.2-RC1, 3.2-RC2, 3.2-RC3, 3.2-RC4

Minimized code

@main def main(): Unit =
  println(summon[Sum[Minus[Succ[Zero]], Minus[Succ[Zero]]] =:= Minus[Succ[Succ[Zero]]]])

sealed trait IntT
sealed trait NatT extends IntT
final case class Zero() extends NatT
final case class Succ[+N <: NatT](n: N) extends NatT
final case class Minus[+N <: Succ[NatT]](n: N) extends IntT

type NatSum[X <: NatT, Y <: NatT] <: NatT = Y match
  case Zero => X
  case Succ[y] => NatSum[Succ[X], y]

type NatDif[X <: NatT, Y <: NatT] <: IntT = Y match
  case Zero => X
  case Succ[y] => X match
    case Zero => Minus[Y]
    case Succ[x] => NatDif[x, y]

type Sum[X <: IntT, Y <: IntT] <: IntT = Y match
  case Zero => X
  case Minus[y] => X match
    case Minus[x] => Minus[NatSum[x, y]]
    case _ => NatDif[X, y]
  case _ => X match
    case Minus[x] => NatDif[Y, x]
    case _ => NatSum[X, Y]

Output

Cannot prove that Minus[NatSum[Succ[Succ[Zero]], NatT]] =:= Minus[Succ[Succ[Zero]]].

Note: a match type could not be fully reduced:

  trying to reduce  NatSum[Succ[Succ[Zero]], NatT]
  failed since selector  NatT
  does not match  case Zero => Succ[Succ[Zero]]
  and cannot be shown to be disjoint from it either.
  Therefore, reduction cannot advance to the remaining case

    case Succ[y] => NatSum[Succ[Succ[Succ[Zero]]], y]

Expectation

Code should compile, as in Scala 3.1.3.

Issue Analytics

State:
Created a year ago
Comments:13 (10 by maintainers)

Top GitHub Comments

1reaction

dwijnandcommented, Oct 25, 2022

We purposely backed out an overly restricting change because of the code that failed to compile because of it. So there was one patch version where it compiled and then not. So not a “regression” as I see it.

1reaction

smartercommented, Oct 18, 2022

We ruled out type avoidance (turning it off completely doesn’t make a difference), the issue does go away if canWidenAbstract is set to false in https://github.com/lampepfl/dotty/blob/96d4ccdcfc3e0ff4d62e969374b92b6db590ccd3/compiler/src/dotty/tools/dotc/core/TypeComparer.scala#L3139

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