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Fix false exhaustivity warning for seq match #23968

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Merged
merged 1 commit into from
Oct 23, 2025
+6 −1
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Fix false exhaustivity warning for seq match #23968

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -362,7 +362,7 @@ object SpaceEngine {
val funRef = fun1.tpe.asInstanceOf[TermRef]
if (fun.symbol.name == nme.unapplySeq)
val (arity, elemTp, resultTp) = unapplySeqInfo(fun.tpe.widen.finalResultType, fun.srcPos)
if fun.symbol.owner == defn.SeqFactoryClass && pat.tpe.hasClassSymbol(defn.ListClass) then
if fun.symbol.owner == defn.SeqFactoryClass && toUnderlying(pat.tpe).dealias.derivesFrom(defn.ListClass) then
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@hamzaremmal hamzaremmal Oct 23, 2025 •
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I'm a bit surprised that the Space engine knows about specifics of the stdlib.

// The exhaustivity and reachability logic already handles decomposing sum types (into its subclasses)
// and product types (into its components). To get better counter-examples for patterns that are of type
// List (or a super-type of list, like LinearSeq) we project them into spaces that use `::` and Nil.
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5 changes: 5 additions & 0 deletions tests/pos/i23928.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
object Test {
Nil match {
case Seq(xs*) => 42
}
}