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Refine parameter adaptation logic for arrays #23591

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bracevac wants to merge 1 commit into scala:main

from dotty-staging:ob-fix-23179

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Refine parameter adaptation logic for arrays #23591

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Refine parameter adaptation logic for arrays

bracevac Jul 23, 2025

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9 changes: 7 additions & 2 deletions compiler/src/dotty/tools/dotc/transform/Erasure.scala

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -483,8 +483,13 @@ object Erasure {
		def sameClass(tp1: Type, tp2: Type) = tp1.classSymbol == tp2.classSymbol

		val paramAdaptationNeeded =
	implParamTypes.lazyZip(samParamTypes).exists((implType, samType) =>
	!sameClass(implType, samType) && !autoAdaptedParam(implType))
	implParamTypes.lazyZip(samParamTypes).exists: (implType, samType) =>
	!sameClass(implType, samType) && !autoAdaptedParam(implType)
	\|\| (samType, implType).match {
	case (defn.ArrayOf(_), defn.ArrayOf(_)) => false
	case (defn.ArrayOf(_), _) => true // see #23179
Copy link Member @sjrd sjrd Jul 25, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Shouldn't there be the reverse order as well (`(_, defn.ArrayOf(_))`)? Like if the SAM takes an `Array[T]` where `T <: AnyRef` but the lambda is specialized to `T = String`, for example? Copy link Contributor Author @bracevac bracevac Jul 25, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. I'm unable to construct such a reversed example that will be accepted by the compiler. sjrd reacted with thumbs up emoji Copy link Member @lrytz lrytz Jul 25, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Do we need something similar for the result type (`resultAdaptationNeeded`)? Copy link Contributor Author @bracevac bracevac Jul 25, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Not sure, I also fail here to construct an example that would cause a run time crash (which is of course weak evidence).
	case _ => false
	}
		val resultAdaptationNeeded =
		!sameClass(implResultType, samResultType) && !autoAdaptedResult

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8 changes: 8 additions & 0 deletions tests/run/i23179.scala

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Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,8 @@
	object Test {
	trait A { def f(a: Array[AnyRef]): Any }
	def g(a: A) = a.f(Array.empty[AnyRef])

	def main(args: Array[String]): Unit = {
	g((x: Array[? >: AnyRef]) => x.headOption)
	}
	}

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