Array erasure: Better Java and Scala 2 compat #11846
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We sometimes need to erase `Array[T]` to `Object` instead of `Array[erasure(T)]` in method signatures, this happens when both primitive and reference arrays, or two different sort of primitive arrays could be passed as arguments because the lub of those types on the JVM is `Object`. But before this commit, we additionally erased Arrays whose element type is upper-bounded by a universal trait to Object (like `Array[_ <: Serializable]`), this isn't necessary since primitives do not extend those traits (except for compiler fictions like `Singleton`) and derived value classes in arrays are always boxed. Since having matching Scala 2 and 3 erasure is a lost cause (cf scala#11603), this commit align ourselves with Java which improves Java interop (because we can emit more accurate Java generic signatures) and should let us simplify erasure more in the future. It also turns out than even before this commit, we did not match Scala 2 erasure perfectly since we erase `Array[_ <: Int]` to `Array[Int]` whereas Scala 2 erases to `Object` (this is important since we want `IArray[Int]` to be erased to `Array[Int]`), so we need to special case Scala 2 array erasure anyway to handle this. This commit renames `isUnboundedGeneric` to `isGenericArrayElement` since the former was somewhat misleading, `T <: String | Any` is bounded, but `Array[T]` cannot be represented with a specific JVM array type.
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@bishabosha Like with #11808, this PR will require some adaptation in Scala 2 to deal with the changed erasure rules, the relevant code in Scala 2 is probably https://github.com/scala/scala/blob/107c72718682024c0494fa81c58ad2301c18aae9/src/reflect/scala/reflect/internal/transform/Erasure.scala#L37 |
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thank you for the tip |
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We sometimes need to erase
Array[T]toObjectinstead ofArray[erasure(T)]in method signatures, this happens when bothprimitive and reference arrays, or two different sort of primitive
arrays could be passed as arguments because the lub of those types on
the JVM is
Object. But before this commit, we additionally erasedArrays whose element type is upper-bounded by a universal trait to
Object (like
Array[_ <: Serializable]), this isn't necessary sinceprimitives do not extend those traits (except for compiler fictions like
Singleton) and derived value classes in arrays are always boxed.Since having matching Scala 2 and 3 erasure is a lost cause (cf #11603),
this commit align ourselves with Java which improves Java
interop (because we can emit more accurate Java generic signatures) and
should let us simplify erasure more in the future.
It also turns out than even before this commit, we did not match Scala 2
erasure perfectly since we erase
Array[_ <: Int]toArray[Int]whereas Scala 2 erases to
Object(this is important since we wantIArray[Int]to be erased toArray[Int]), so we need to special caseScala 2 array erasure anyway to handle this.
This commit renames
isUnboundedGenerictoisGenericArrayElementsince the former was somewhat misleading,
T <: String | Anyisbounded, but
Array[T]cannot be represented with a specific JVM arraytype.