Fix #7807: Add case for MatchTypes to ApproximatingTypeMap

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3966,7 +3966,6 @@ object Types {
    *  and `X_1,...X_n` are the type variables bound in `patternType`
    */
   abstract case class MatchType(bound: Type, scrutinee: Type, cases: List[Type]) extends CachedProxyType with ValueType {
-
     def derivedMatchType(bound: Type, scrutinee: Type, cases: List[Type])(implicit ctx: Context): MatchType =
       if (bound.eq(this.bound) && scrutinee.eq(this.scrutinee) && cases.eqElements(this.cases)) this
       else MatchType(bound, scrutinee, cases)
@@ -4945,6 +4944,15 @@ object Types {
     override protected def derivedWildcardType(tp: WildcardType, bounds: Type): WildcardType =
       tp.derivedWildcardType(rangeToBounds(bounds))
 
+    override protected def derivedMatchType(tp: MatchType, bound: Type, scrutinee: Type, cases: List[Type]): Type =
+      bound match
+        case Range(lo, hi) =>
+          range(derivedMatchType(tp, lo, scrutinee, cases), derivedMatchType(tp, hi, scrutinee, cases))
+        case _ =>
+          scrutinee match
+            case Range(lo, hi) => range(bound.bounds.lo, bound.bounds.hi)
+            case _ => tp.derivedMatchType(bound, scrutinee, cases)
+
     override protected def derivedSkolemType(tp: SkolemType, info: Type): Type = info match {
       case Range(lo, hi) =>
         range(tp.derivedSkolemType(lo), tp.derivedSkolemType(hi))

tests/pos/i7807.scala

@@ -0,0 +1,3 @@
+object Test with
+
+  def flip: (x: 0 | 1) => x.type match { case 0 => 1 case 1 => 0 } = ???