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Always use baseType when constraining patternTp with scrutineeTp #20032

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Apr 6, 2024
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Always use baseType when constraining patternTp with scrutineeTp #20032

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12 changes: 5 additions & 7 deletions compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -200,8 +200,8 @@ trait PatternTypeConstrainer { self: TypeComparer =>
*
* This function expects to receive two types (scrutinee and pattern), both
* of which have class symbols, one of which is derived from another. If the
* type "being derived from" is an applied type, it will 1) "upcast" the
* deriving type to an applied type with the same constructor and 2) infer
* type "being derived from" is an applied type, it will 1) "upcast" both
* types to an applied type with the same constructor and 2) infer
* constraints for the applied types' arguments that follow from both
* types being inhabited by one value (the scrutinee).
*
Expand Down Expand Up @@ -252,11 +252,9 @@ trait PatternTypeConstrainer { self: TypeComparer =>
val scrutineeCls = scrutineeTp.classSymbol

// NOTE: we already know that there is a derives-from relationship in either direction
val upcastPattern =
patternCls.derivesFrom(scrutineeCls)

val pt = if upcastPattern then patternTp.baseType(scrutineeCls) else patternTp
val tp = if !upcastPattern then scrutineeTp.baseType(patternCls) else scrutineeTp
val base = if patternCls.derivesFrom(scrutineeCls) then scrutineeCls else patternCls
val pt = patternTp.baseType(base)
val tp = scrutineeTp.baseType(base)

val assumeInvariantRefinement =
migrateTo3 || forceInvariantRefinement || refinementIsInvariant(patternTp)
Expand Down
29 changes: 29 additions & 0 deletions tests/pos/i19706.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,29 @@

import scala.compiletime.ops.string.{Length, Matches, Substring}

def emptyContext(): Unit =
summon[Decoded["Tuple(0, EmptyTuple)"] =:= 0 *: EmptyTuple]

type Decoded[S <: String] = Matches[S, "Tuple(.+, .+)"] match
case true => Parsed[Substring[S, 6, 19], 0, ""] match
case (h, t) => Decoded["0"] *: EmptyTuple
case false => 0

type Parsed[S <: String, I <: Int, A <: String] <: (String, String) = Matches[S, "other"] match
case true => I match
case 1 => ("", "")
case _ => Parsed[Substring[S, 1, Length[S]], I, ""]
case false => ("0", "EmptyTuple")


object Minimization:

type Cond[B <: Boolean] <: Tuple2[String, String] = B match
case true => ("", "")
case false => ("a", "b")

type Decoded[B <: Boolean] = Cond[B] match
case (h1, _) => Int

val _: Decoded[false] = 1

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