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Fix #17467: Limit isNullable widening to stable TermRefs; remove under explicit nulls. (#17470)
The Scala language specification has a peculiar clause about the nullness of singleton types of the form `path.type`. It says that `Null <:< path.type` if the *underlying* type `U` of `path` is nullable itself. The previous implementation of that rule was overly broad, as it indiscrimately widened all types. This resulted in problematic subtyping relationships like `Null <:< "foo"`. We do not widen anymore. Instead, we specifically handle `TermRef`s of stable members, which are how dotc represents singleton types. We also have a rule for `Null <:< null`, which is necessary for pattern matching exhaustivity to keep working in the presence of nulls. --- #### Under explicit nulls, remove the rule Null <:< x.type. The specified rule that `Null <:< x.type` when the underlying type `U` of `x` is nullable is dubious to begin with. Under explicit nulls, it becomes decidedly out of place. We now disable that rule under `-Yexplicit-nulls`.
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‎compiler/src/dotty/tools/dotc/core/TypeComparer.scala‎

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@@ -943,16 +943,24 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
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// However, `null` can always be a value of `T` for Java side.
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// So the best solution here is to let `Null` be a subtype of non-primitive
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// value types temporarily.
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def isNullable(tp: Type): Boolean = tp.widenDealias match
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def isNullable(tp: Type): Boolean = tp.dealias match
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case tp: TypeRef =>
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val tpSym = tp.symbol
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ctx.mode.is(Mode.RelaxedOverriding) && !tpSym.isPrimitiveValueClass ||
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tpSym.isNullableClass
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case tp: TermRef =>
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// https://scala-lang.org/files/archive/spec/2.13/03-types.html#singleton-types
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// A singleton type is of the form p.type. Where p is a path pointing to a value which conforms to
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// scala.AnyRef [Scala 3: which scala.Null conforms to], the type denotes the set of values consisting
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// of null and the value denoted by p (i.e., the value v for which v eq p). [Otherwise,] the type
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// denotes the set consisting of only the value denoted by p.
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!ctx.explicitNulls && isNullable(tp.underlying) && tp.isStable
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case tp: RefinedOrRecType => isNullable(tp.parent)
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case tp: AppliedType => isNullable(tp.tycon)
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case AndType(tp1, tp2) => isNullable(tp1) && isNullable(tp2)
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case OrType(tp1, tp2) => isNullable(tp1) || isNullable(tp2)
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case AnnotatedType(tp1, _) => isNullable(tp1)
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case ConstantType(c) => c.tag == Constants.NullTag
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case _ => false
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val sym1 = tp1.symbol
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(sym1 eq NothingClass) && tp2.isValueTypeOrLambda ||

‎tests/explicit-nulls/neg/i17467.check‎

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@@ -0,0 +1,31 @@
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-- [E007] Type Mismatch Error: tests/explicit-nulls/neg/i17467.scala:4:22 ----------------------------------------------
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4 | val a2: a1.type = null // error
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| ^^^^
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| Found: Null
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| Required: (a1 : String)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (a1 : String)
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|
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| longer explanation available when compiling with `-explain`
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-- [E007] Type Mismatch Error: tests/explicit-nulls/neg/i17467.scala:7:22 ----------------------------------------------
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7 | val b2: b1.type = null // error
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| ^^^^
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| Found: Null
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| Required: (b1 : String | Null)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (b1 : String | Null)
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|
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| longer explanation available when compiling with `-explain`
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-- [E172] Type Error: tests/explicit-nulls/neg/i17467.scala:8:28 -------------------------------------------------------
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8 | summon[Null <:< b1.type] // error
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| ^
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| Cannot prove that Null <:< (b1 : String | Null).
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-- [E007] Type Mismatch Error: tests/explicit-nulls/neg/i17467.scala:14:22 ---------------------------------------------
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14 | val c2: c1.type = null // error
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| ^^^^
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| Found: Null
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| Required: (c1 : Null)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (c1 : Null)
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|
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| longer explanation available when compiling with `-explain`

‎tests/explicit-nulls/neg/i17467.scala‎

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object Test:
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def test(): Unit =
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val a1: String = "foo"
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val a2: a1.type = null // error
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val b1: String | Null = "foo"
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val b2: b1.type = null // error
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summon[Null <:< b1.type] // error
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/* The following would be sound, but it would require a specific subtyping
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* rule (and implementation code) for debatable value. So it is an error.
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*/
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val c1: Null = null
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val c2: c1.type = null // error
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end test
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end Test

‎tests/neg/i17467.check‎

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-- [E007] Type Mismatch Error: tests/neg/i17467.scala:6:20 -------------------------------------------------------------
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6 | val b1: "foo" = null // error
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| ^^^^
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| Found: Null
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| Required: ("foo" : String)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than ("foo" : String)
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|
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| longer explanation available when compiling with `-explain`
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-- [E007] Type Mismatch Error: tests/neg/i17467.scala:9:22 -------------------------------------------------------------
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9 | val c2: c1.type = null // error
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| ^^^^
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| Found: Null
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| Required: (c1 : ("foo" : String))
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (c1 : ("foo" : String))
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|
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| longer explanation available when compiling with `-explain`
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-- [E007] Type Mismatch Error: tests/neg/i17467.scala:17:22 ------------------------------------------------------------
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17 | val e2: e1.type = null // error
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| ^^^^
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| Found: Null
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| Required: (e1 : MyNonNullable)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (e1 : MyNonNullable)
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|
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| longer explanation available when compiling with `-explain`
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-- [E172] Type Error: tests/neg/i17467.scala:19:26 ---------------------------------------------------------------------
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19 | summon[Null <:< "foo"] // error
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| ^
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| Cannot prove that Null <:< ("foo" : String).
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-- [E007] Type Mismatch Error: tests/neg/i17467.scala:21:23 ------------------------------------------------------------
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21 | val f1: Mod.type = null // error
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| ^^^^
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| Found: Null
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| Required: Test.Mod.type
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than Test.Mod.type
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|
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| longer explanation available when compiling with `-explain`
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-- [E083] Type Error: tests/neg/i17467.scala:24:12 ---------------------------------------------------------------------
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24 | val g2: g1.type = null // error // error
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| ^^^^^^^
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| (g1 : AnyRef) is not a valid singleton type, since it is not an immutable path
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|
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| longer explanation available when compiling with `-explain`
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-- [E007] Type Mismatch Error: tests/neg/i17467.scala:24:22 ------------------------------------------------------------
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24 | val g2: g1.type = null // error // error
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| ^^^^
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| Found: Null
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| Required: (g1 : AnyRef)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (g1 : AnyRef)
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|
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| longer explanation available when compiling with `-explain`
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-- [E007] Type Mismatch Error: tests/neg/i17467.scala:33:24 ------------------------------------------------------------
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33 | def me: this.type = null // error
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| ^^^^
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| Found: Null
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| Required: (Bar.this : Test.Bar)
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| Note that implicit conversions were not tried because the result of an implicit conversion
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| must be more specific than (Bar.this : Test.Bar)
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|
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| longer explanation available when compiling with `-explain`

‎tests/neg/i17467.scala‎

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object Test:
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def test(): Unit =
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val a1: String = "foo"
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val a2: a1.type = null // OK
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val b1: "foo" = null // error
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val c1: "foo" = "foo"
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val c2: c1.type = null // error
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type MyNullable = String
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val d1: MyNullable = "foo"
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val d2: d1.type = null // OK
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type MyNonNullable = Int
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val e1: MyNonNullable = 5
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val e2: e1.type = null // error
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summon[Null <:< "foo"] // error
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val f1: Mod.type = null // error
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var g1: AnyRef = "foo"
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val g2: g1.type = null // error // error
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val h1: Null = null
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val h2: h1.type = null
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end test
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object Mod
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class Bar:
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def me: this.type = null // error
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end Test

‎tests/run/t6443b.scala‎

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@@ -2,7 +2,10 @@ trait A {
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type D >: Null <: C
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def foo(d: D)(d2: d.type): Unit
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trait C {
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def bar: Unit = foo(null)(null)
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def bar: Unit = {
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val nul = null
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foo(nul)(nul)
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}
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}
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}
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object B extends A {

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