Enhancement: Add missing case for comparison of capturing types.

Add a case for cases like

    x.type <:< {r} (A | x.type)

where `x` captures `r`.

# Conflicts:
#	tests/pos-with-compiler-cc/dotc/core/Types.scala

Author: odersky

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

@@ -826,7 +826,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             if refs1.isAlwaysEmpty then recur(tp1, parent2)
             else subCaptures(refs1, refs2, frozenConstraint).isOK
               && sameBoxed(tp1, tp2, refs1)
-              && recur(tp1.widen.stripCapturing, parent2)
+              && (recur(tp1.widen.stripCapturing, parent2)
+                 || tp1.isInstanceOf[SingletonType] && recur(tp1, parent2)
+                    // this alternative is needed in case the right hand side is a
+                    // capturing type that contains the lhs as an |-alternative.
+                 )
           catch case ex: AssertionError =>
             println(i"assertion failed while compare captured $tp1 <:< $tp2")
             throw ex

tests/pos-custom-args/captures/cmp-singleton.scala

@@ -0,0 +1,10 @@
+class Denotation
+abstract class SingleDenotation extends Denotation
+def goRefined: Denotation =
+  val foo: Denotation = ???
+  val joint = foo
+  joint match
+    case joint: SingleDenotation =>
+      joint
+    case _ =>
+      joint