Type match with a match type when a match type is expected

Author: mbovel

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1553,7 +1553,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           case otherTpe => otherTpe.widen
         /** Extractor for match types hidden behind an AppliedType/MatchAlias */
         object MatchTypeInDisguise {
-          def unapply(tp: AppliedType)(using Context): Option[MatchType] = tp match {
+          def unapply(tp: Type)(using Context): Option[MatchType] = tp match {
             case AppliedType(tycon: TypeRef, args) =>
               tycon.info match {
                 case MatchAlias(alias) =>
@@ -1563,6 +1563,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                   }
                 case _ => None
               }
+            case mt: MatchType => Some(mt)
             case _ => None
           }
         }

tests/pos/match-type-inference.scala

@@ -0,0 +1,22 @@
+type IsString[T <: Any] = T match {
+  case String => true
+  case _ => false
+}
+
+def isString(x: Any): IsString[x.type] = x match
+  case _: String => true
+  case _ => false
+
+def isString2(x: Any): x.type match {
+  case String => true
+  case _ => false
+} = x match
+  case _: String => true
+  case _ => false
+
+def isString3[T](x: T): T match {
+  case String => true
+  case _ => false
+} = x match
+  case _: String => true
+  case _ => false