Fix #13757: Explicitly disallow higher-kinded scrutinees of match types.

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -192,6 +192,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnusedNonUnitValueID // errorNumber 176
   case ConstrProxyShadowsID // errorNumber 177
   case MissingArgumentListID // errorNumber: 178
+  case MatchTypeScrutineeCannotBeHigherKindedID // errorNumber: 179
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2878,3 +2878,8 @@ class UnusedNonUnitValue(tp: Type)(using Context)
     def kind = MessageKind.PotentialIssue
     def msg(using Context) = i"unused value of type $tp"
     def explain(using Context) = ""
+
+class MatchTypeScrutineeCannotBeHigherKinded(tp: Type)(using Context)
+  extends TypeMsg(MatchTypeScrutineeCannotBeHigherKindedID) :
+    def msg(using Context) = i"the scrutinee of a match type cannot be higher-kinded"
+    def explain(using Context) = ""

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

@@ -2214,8 +2214,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       if (tree.bound.isEmpty && isFullyDefined(pt, ForceDegree.none)) TypeTree(pt)
       else typed(tree.bound)
     val sel1 = typed(tree.selector)
+    val sel1Tpe = sel1.tpe
+    if sel1Tpe.isLambdaSub then
+      report.error(MatchTypeScrutineeCannotBeHigherKinded(sel1Tpe), sel1.srcPos)
     val pt1 = if (bound1.isEmpty) pt else bound1.tpe
-    val cases1 = tree.cases.mapconserve(typedTypeCase(_, sel1.tpe, pt1))
+    val cases1 = tree.cases.mapconserve(typedTypeCase(_, sel1Tpe, pt1))
     assignType(cpy.MatchTypeTree(tree)(bound1, sel1, cases1), bound1, sel1, cases1)
   }
 

tests/neg/i13757-match-type-anykind.scala

@@ -0,0 +1,16 @@
+object Test:
+  type AnyKindMatchType1[X <: AnyKind] = X match // error: the scrutinee of a match type cannot be higher-kinded // error
+    case Option[a] => Int
+
+  type AnyKindMatchType2[X <: AnyKind] = X match // error: the scrutinee of a match type cannot be higher-kinded
+    case Option => Int // error: Missing type parameter for Option
+
+  type AnyKindMatchType3[X <: AnyKind] = X match // error: the scrutinee of a match type cannot be higher-kinded // error
+    case _ => Int
+
+  type AnyKindMatchType4[X <: Option] = X match // error // error: the scrutinee of a match type cannot be higher-kinded // error
+    case _ => Int
+
+  type AnyKindMatchType5[X[_]] = X match // error: the scrutinee of a match type cannot be higher-kinded // error
+    case _ => Int
+end Test