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Fix isEffectivelySingleton #20486

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merged 1 commit into from
May 28, 2024
Merged

Fix isEffectivelySingleton #20486

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3 changes: 1 addition & 2 deletions compiler/src/dotty/tools/dotc/core/Types.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -329,13 +329,12 @@ object Types extends TypeUtils {
def isSingleton(using Context): Boolean = dealias.isInstanceOf[SingletonType]

/** Is this type a (possibly aliased) singleton type or a type proxy
* or Or/And type known to be a singleton type?
* or an AndType where one operand is effectively a singleton?
*/
def isEffectivelySingleton(using Context): Boolean = dealias match
case tp: SingletonType => true
case tp: TypeProxy => tp.superType.isEffectivelySingleton
case AndType(tpL, tpR) => tpL.isEffectivelySingleton || tpR.isEffectivelySingleton
case OrType(tpL, tpR) => tpL.isEffectivelySingleton && tpR.isEffectivelySingleton
case _ => false

/** Is this upper-bounded by a (possibly aliased) singleton type?
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13 changes: 13 additions & 0 deletions tests/neg/i20474.scala
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@@ -0,0 +1,13 @@
class A
class B extends A

def f(a: A, c: A) =
val b1: a.type = a
val b2: a.type & B = a.asInstanceOf[a.type & B]
val b3: c.type & A = c
val b4: a.type | c.type = c
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Can you add the test below for OrType as well?

val b5: a.type | a.type = a

val d5: b5.type = a // ok

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@odersky odersky May 28, 2024 •
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I don't think that will work. Have you tried it? I see no strong reason to make it work.

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It works in 3.4.2: https://scastie.scala-lang.org/A6PehvQvRhW2sUPVVtHiuA
I think the atoms is doing all these checks.

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Ah, yes, atoms would do it. Bit it's an extreme corner case. Not sure we want to make sure it works in the general case. Because in this case we'd also have to accept

val b6: (a.type & A) | a.type
val d6: b6.type = a // ok

and I am not sure atoms would help us here.


val d1: b1.type = a
val d2: b2.type = a // ok
val d3: b3.type = a // error
val d4: b4.type = a // error