[WIP] Handle match types reducing to ErrorType in TypeComparer

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

@@ -543,7 +543,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         recur(tp1.parent, tp2)
       case tp1: MatchType =>
         val reduced = tp1.reduced
-        if reduced.exists then
+        if reduced.exists && !reduced.isError then
           recur(reduced, tp2) && recordGadtUsageIf { MatchType.thatReducesUsingGadt(tp1) }
         else thirdTry
       case _: FlexType =>
@@ -786,7 +786,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         either(recur(tp1, tp21), recur(tp1, tp22)) || fourthTry
       case tp2: MatchType =>
         val reduced = tp2.reduced
-        if reduced.exists then
+        if reduced.exists && !reduced.isError then
           recur(tp1, reduced) && recordGadtUsageIf { MatchType.thatReducesUsingGadt(tp2) }
         else
           fourthTry

tests/neg/i19949.scala

@@ -0,0 +1,9 @@
+
+trait T[N]:
+  type M = N match
+    case 0 => Any
+
+val t: T[Double] = new T[Double] {}
+val x: t.M = "hello" // error
+
+val z: T[Double]#M = "hello" // error

tests/pos/i19950.scala

@@ -0,0 +1,10 @@
+
+trait Apply[F[_]]:
+  extension [T <: NonEmptyTuple](tuple: T)(using toMap: Tuple.IsMappedBy[F][T])
+    def mapN[B](f: Tuple.InverseMap[T, F] => B): F[B] = ???
+
+given Apply[Option] = ???
+given Apply[List] = ???
+given Apply[util.Try] = ???
+
+@main def Repro = (Option(1), Option(2), Option(3)).mapN(_ + _ + _)