`@unchecked` tuple destructuring is now ignored

[WojciechMazur]

Based on the OpenCB failures in:

• dfianthdl/dfhdl - build logs

Compiler version

Last good nightly: 3.8.4-RC1-bin-20260227-4e9c706-NIGHTLY
First bad nightly: 3.8.4-RC1-bin-20260228-f302695-NIGHTLY

Bisect points to: 26cf713 (unlikely

Nightly bisect range: 4e9c706...f302695

Minimized code

import scala.unchecked

sealed trait Tree
final class DefDef extends Tree

object Repro:
  val trees: List[Tree] = Nil

  val (allDefs: List[DefDef] @unchecked, allStepBlocks: List[Tree]) =
    trees.partition {
      case _: DefDef => true
      case _ => false
    }

Output

$ scala-cli compile test.scala -S 3.8.4-RC1-bin-20260228-f302695-NIGHTLY --server=false
-- [E007] Type Mismatch Error: test.scala:9:39
9 |  val (allDefs: List[DefDef] @unchecked, allStepBlocks: List[Tree]) =
  |                                       ^
  |                                     Found:    (Repro.$2$._1 : List[Tree])
  |                                     Required: List[DefDef] @unchecked
  |
  | longer explanation available when compiling with `-explain`
1 error found
Compilation failed

Expectation

This should compile successfully, at most with the same narrowing warning emitted by 3.8.3-RC2 and 3.8.4-RC1-bin-20260227-4e9c706-NIGHTLY.

[SolalPirelli]

Probably my bad due to the tuple desugaring change, looking into it

[SolalPirelli]

Per Seb's comment in the linked PR this shouldn't work to begin with :/

[SethTisue]

List[DefDef @unchecked] is the form I would have written and I would expect to still work

List is runtime-checkable, it's the DefDef part that isn't

[SolalPirelli]

As just discussed in the Core meeting, @SethTisue is correct here and the code in the issue should not have compiled to begin with.

[tgodzik]

Looks like this would work:

  val (allDefs: List[DefDef] , allStepBlocks: List[Tree]) =
    trees.partition {
      case _: DefDef => true
      case _ => false
    }: @unchecked

Adding @unchecked on DefDef didn't seem to help

[SethTisue]

Adding @unchecked on DefDef didn't seem to help

Doesn't it avoid the "Type Mismatch Error"?

With List[DefDef @unchecked] I get:

-- Warning: --------------------------------------------------------------------
10 |    trees.partition {
   |    ^
   |pattern's type List[DefDef @unchecked] is more specialized than the right hand side expression's type List[Tree]
   |
   |If the narrowing is intentional, this can be communicated by adding `.runtimeChecked` after the expression,
   |which may result in a MatchError at runtime.
   |

This is different from the Scala 2.13.18 behavior, where there is no warning.

So that's a different issue, and it seems like a regression.