Backport "Improve recursive decompose prefix fix" to LTS (#20818)

Backports #19375 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -613,7 +613,7 @@ object SpaceEngine {
       case OrType(tp1, tp2)                            => List(tp1, tp2)
       case tp if tp.isRef(defn.BooleanClass)           => List(ConstantType(Constant(true)), ConstantType(Constant(false)))
       case tp if tp.isRef(defn.UnitClass)              => ConstantType(Constant(())) :: Nil
-      case tp @ NamedType(Parts(parts), _)             => parts.map(tp.derivedSelect)
+      case tp @ NamedType(Parts(parts), _)             => if parts.exists(_ eq tp) then ListOfNoType else parts.map(tp.derivedSelect)
       case _: SingletonType                            => ListOfNoType
       case tp if tp.classSymbol.isAllOf(JavaEnumTrait) => tp.classSymbol.children.map(_.termRef)
         // the class of a java enum value is the enum class, so this must follow SingletonType to not loop infinitely

tests/pos/i19031.ci-reg1.scala → tests/warn/i19031.ci-reg1.scala

@@ -1,5 +1,3 @@
-//> using options -Werror
-
 sealed trait Mark[T]
 
 trait Foo[T]

tests/pos/i19031.ci-reg2.scala → tests/warn/i19031.ci-reg2.scala

@@ -1,5 +1,3 @@
-//> using options -Werror
-
 trait Outer:
   sealed trait Foo
   case class  Bar1() extends Foo

tests/pos/i19031.scala → tests/warn/i19031.scala

@@ -1,5 +1,3 @@
-//> using options -Werror
-
 sealed trait A:
   class B extends A
 

tests/warn/i19031b.scala

@@ -0,0 +1,10 @@
+sealed trait A:
+  class B extends A
+
+class C extends A
+
+class Test:
+  def t1(a: A): Boolean =
+    a match
+      case b: A#B => true
+      case c: C   => true