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Backport "Fix #20897: Make Nothing ⋔ Nothing, as per spec." to LTS #22077

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Dec 3, 2024
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Backport "Fix #20897: Make Nothing ⋔ Nothing, as per spec." to LTS #22077

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12 changes: 12 additions & 0 deletions compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2881,6 +2881,12 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
true
case (_, _: HKLambda) =>
true
/* Nothing is not a class type in the spec but dotc represents it as if it were one.
* Get it out of the way early to avoid mistakes (see for example #20897).
* Nothing ⋔ T and T ⋔ Nothing for all T.
*/
case (tp1, tp2) if tp1.isExactlyNothing || tp2.isExactlyNothing =>
true
case (tp1: OrType, _) =>
provablyDisjoint(tp1.tp1, tp2) && provablyDisjoint(tp1.tp2, tp2)
case (_, tp2: OrType) =>
Expand All @@ -2891,6 +2897,12 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
case (_, tp2: AndType) =>
!(tp2 <:< tp1)
&& (provablyDisjoint(tp1, tp2.tp2) || provablyDisjoint(tp1, tp2.tp1))
/* Handle AnyKind now for the same reason as Nothing above: it is not a real class type.
* Other than the rules with Nothing, unions and intersections, there is structurally
* no rule such that AnyKind ⋔ T or T ⋔ AnyKind for any T.
*/
case (tp1, tp2) if tp1.isDirectRef(AnyKindClass) || tp2.isDirectRef(AnyKindClass) =>
false
case (tp1: NamedType, _) if gadtBounds(tp1.symbol) != null =>
provablyDisjoint(gadtBounds(tp1.symbol).uncheckedNN.hi, tp2)
|| provablyDisjoint(tp1.superTypeNormalized, tp2)
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2 changes: 1 addition & 1 deletion compiler/test/dotc/pos-test-pickling.blacklist
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Expand Up @@ -57,6 +57,7 @@ i15158.scala
i15155.scala
i15827.scala
i18211.scala
i20897.scala

# Opaque type
i5720.scala
Expand Down Expand Up @@ -112,4 +113,3 @@ i15525.scala
i19955a.scala
i19955b.scala
i20053b.scala

10 changes: 10 additions & 0 deletions tests/pos/i20897.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
object Test:
type Disj[A, B] =
A match
case B => true
case _ => false

def f(a: Disj[1 | Nothing, 2 | Nothing]): Unit = ()

val t = f(false)
end Test
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