Changed type constraint behaviour when pattern matching with unions

[WojciechMazur]

Based on OpenCB failure in in outr/lightdb

Compiler version

Last good release: 3.7.1-RC1-bin-20250323-451fdcd-NIGHTLY
First bad release: 3.7.1-RC1-bin-20250325-701fa8a-NIGHTLY

Bisect points to either 451fdcd or 701fa8a (most likekly)

Minimized code

trait Document[Doc <: Document[Doc]]
sealed trait Conversion[Doc, V]

object Conversion {
  case class Doc[Doc <: Document[Doc]]() extends Conversion[Doc, Doc]
  case class Converted[Doc <: Document[Doc], T](f: Doc => T) extends Conversion[Doc, T]
}

def Test[Doc <: Document[Doc], V](conversion: Conversion[Doc, V]) = 
  val fails = conversion match 
    case Conversion.Doc() | Conversion.Converted(_) => ??? // error
  val works = conversion match 
    case Conversion.Doc() => ???
    case Conversion.Converted(_) => ???

Output

-- [E057] Type Mismatch Error: /Users/wmazur/projects/scala/sandbox/test.scala:12:9 ---------------------------------------------------------------------------------------------------------
12 |    case Conversion.Doc() | Conversion.Converted(_) => ??? // error
   |         ^
   |         Type argument V does not conform to upper bound Document[V]
   |
   | longer explanation available when compiling with `-explain`

Expectation

Should probably compile without error or fail in both cases

[mbovel]

Further minimized:

trait Document[Doc <: Document[Doc]]
sealed trait Conversion[Doc, V]

case class C[Doc <: Document[Doc]]() extends Conversion[Doc, Doc]

def Test[Doc <: Document[Doc], V](conversion: Conversion[Doc, V]) =
  conversion match
    case C() | C() => ??? // error

It type-checks without the union:

trait Document[Doc <: Document[Doc]]
sealed trait Conversion[Doc, V]

case class C[Doc <: Document[Doc]]() extends Conversion[Doc, Doc]

def Test[Doc <: Document[Doc], V](conversion: Conversion[Doc, V]) =
  conversion match
    case C() => ??? // error

I haven't found out yet what differs between the two. The following calls and constraints are the same in both cases:

checkBounds: C.unapply [Doc <: Document[Doc]](x$1: C[Doc]): (true : Boolean)
boundsViolations([V], [ <: Document[Doc]], <notype>)
checkOverlapsBounds(V, V, V,  <: Document[Doc])
check(V, Document[V], upper, Document[V])
 constrained types = 
 bounds = 
 coDeps = ()
 contraDeps = ()
check(Nothing, V, lower, Nothing)
 constrained types = 
 bounds = 
 coDeps = ()
 contraDeps = ()

[mbovel]

I confirm the culprit is #22853; commenting

scala3/compiler/src/dotty/tools/dotc/typer/Typer.scala

Line 2832 in 66d15b2

nestedCtx.gadtState.restore(savedGadt) // Disable GADT reasoning for pattern alternatives

allows the test to pass.

[mbovel]

@Linyxus I will need your help on this one. Could we have a look together one of these days?