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Fix #24997: Avoid unnecessary CHECKCAST in untupled function parameters #25085

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Fix #24997: Avoid unnecessary CHECKCAST in untupled function parameters #25085

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Fix #24997: Avoid unnecessary CHECKCAST in untupled function parameters
aviralgarg05 Jan 23, 2026
0378b70
Improve i24997 test to check lambdas and assert exact CHECKCAST counts
aviralgarg05 Jan 27, 2026
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4 changes: 2 additions & 2 deletions compiler/src/dotty/tools/dotc/ast/Desugar.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -1974,8 +1974,8 @@ object desugar {
if (isGenericTuple) Apply(Select(refOfDef(param), nme.apply), Literal(Constant(n)))
else Select(refOfDef(param), nme.selectorName(n))
val vdefs =
params.zipWithIndex.map {
case (param, idx) =>
params.zipWithIndex.collect {
case (param, idx) if param.name != nme.WILDCARD =>
ValDef(param.name, param.tpt, selector(idx))
.withSpan(param.span)
.withAttachment(UntupledParam, ())
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41 changes: 41 additions & 0 deletions compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2009,6 +2009,47 @@ class DottyBytecodeTests extends DottyBytecodeTest {
assertEquals(expected, clsNode.interfaces.asScala)
}
}

// Test for https://github.com/scala/scala3/issues/24997
// Automatic untupling should not introduce extra CHECKCAST instructions for unused tuple elements
@Test def i24997 = {
val source =
"""class Wrapper[A](value: A):
| def use[B](f: A => B): B = f(value)
|
|class Test:
| def withCase: Int =
| val w = Wrapper((42, "Answer"))
| w.use { case (number, _) => number }
|
| def withoutCase: Int =
| val w = Wrapper((42, "Answer"))
| w.use { (number, _) => number }
|""".stripMargin

checkBCode(source) { dir =>
val clsIn = dir.lookupName("Test.class", directory = false).input
val clsNode = loadClassNode(clsIn)

// Get instructions for both methods
def getCheckCastCount(methodPrefix: String): Int = {
clsNode.methods.asScala.filter(_.name.startsWith(methodPrefix)).map { method =>
instructionsFromMethod(method).count {
case TypeOp(Opcodes.CHECKCAST, _) => true
case _ => false
}
}.sum
}

val withCaseCasts = getCheckCastCount("withCase")
val withoutCaseCasts = getCheckCastCount("withoutCase")

// Both methods should have the same number of CHECKCAST instructions
// (specifically, they should NOT have extra ones for unused wildcard elements)
assertEquals(s"withCase should have 0 CHECKCASTs", 0, withCaseCasts)
assertEquals(s"withoutCase should have 1 CHECKCAST", 1, withoutCaseCasts)
}
}
}

object invocationReceiversTestCode {
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