Make erased capability-safe
#23419
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Make erased capability-safe
#23419
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Since we will force erased expressions to be pure values, they are always initialized.
# Conflicts: # compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
- erasedValue[<ConstantType>] is now considered to be pure - calls of synthetic case class apply are considered pure if the case class is NoInits - Companions of Scala-2 classes Tuple and Some are assumed to be NoInits
natsukagami
reviewed
Jul 3, 2025
| @@ -6,15 +6,15 @@ object App { | |||
| trait A { type L >: Any} | |||
| def upcast(erased a: A)(x: Any): a.L = x | |||
| erased val p: A { type L <: Nothing } = p | |||
| def coerce(x: Any): Int = upcast(p)(x) // error | |||
| def coerce(x: Any): Int = upcast(p)(x) // ok? | |||
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| def coerceInline(x: Any): Int = upcast(compiletime.erasedValue[A {type L <: Nothing}])(x) // error | |||
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| trait B { type L <: Nothing } | |||
| def upcast_dep_parameter(erased a: B)(x: a.L) : Int = x | |||
| erased val q : B { type L >: Any } = compiletime.erasedValue | |||
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Should this be rejected? This should be unsafeErasedValue
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It gets rejected for a different reason before.
| @@ -6,15 +6,15 @@ object App { | |||
| trait A { type L >: Any} | |||
| def upcast(erased a: A)(x: Any): a.L = x | |||
| erased val p: A { type L <: Nothing } = p | |||
| def coerce(x: Any): Int = upcast(p)(x) // error | |||
| def coerce(x: Any): Int = upcast(p)(x) // ok? | |||
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This should be rejected, perhaps by initialization checker?
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Yes, but that's not part of the test. If we turn on the initialization checker we won't make it to erasure to detect the other problems.
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Basically, all that remains is a realizability check. The rest is would be detected if the check is omitted.
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The purpose of
erasedis to give compile-time evidence without requiring a runtime value. The compile time evidence is typically for a type class instance or a capability. The previous design oferasedis not safe in this respect, as any value can be summoned by various means, includingerasedValueexpression of the required type,This PR implements a set of changes to make
eraseda trustable evidence.erased def.erasedvals as late initialized.erasedValue, which, unlikeerasedValue itself,are treated as pure expressions.