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Fix: Prevent GADT reasoning in pattern alternatives #22853

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Merged
merged 1 commit into from
Mar 25, 2025
Merged

Fix: Prevent GADT reasoning in pattern alternatives #22853

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2 changes: 2 additions & 0 deletions compiler/src/dotty/tools/dotc/typer/Typer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2805,8 +2805,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
else
assert(ctx.reporter.errorsReported)
tree.withType(defn.AnyType)
val savedGadt = nestedCtx.gadt
val trees1 = tree.trees.mapconserve(typed(_, pt)(using nestedCtx))
.mapconserve(ensureValueTypeOrWildcard)
nestedCtx.gadtState.restore(savedGadt) // Disable GADT reasoning for pattern alternatives
assignType(cpy.Alternative(tree)(trees1), trees1)
}

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9 changes: 9 additions & 0 deletions tests/neg/gadt-alternatives.scala
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@@ -0,0 +1,9 @@
enum Expr[+T]:
case StringVal(x: String) extends Expr[String]
case IntVal(x: Int) extends Expr[Int]
case IntValAlt(x: Int) extends Expr[Int]
import Expr.*
def eval[T](e: Expr[T]): T = e match
case StringVal(_) | IntVal(_) => "42" // error
def eval1[T](e: Expr[T]): T = e match
case IntValAlt(_) | IntVal(_) => 42 // error // limitation
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