Add missing case to TypeComparer

We now have:

  x.type^{x} <: x.type

Author: odersky

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

@@ -547,6 +547,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           if tp2.isAny then true
           else if compareCaptures(tp1, refs1, tp2, tp2.captureSet)
             || !ctx.mode.is(Mode.CheckBoundsOrSelfType) && tp1.isAlwaysPure
+            || parent1.isSingleton && refs1.elems.forall(parent1 eq _)
           then
             val tp2a =
               if tp1.isBoxedCapturing && !parent1.isBoxedCapturing

tests/neg-custom-args/captures/i23207.scala

@@ -13,4 +13,6 @@ def leak(io: AnyRef^): A =
   val b = new B
   val box = b.getBox
   val a: A = box.x // error
-  b.getBox.x // error
+  val c = b.getBox.x // now OK
+  val _: B^{b} = c
+  c

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

@@ -0,0 +1,6 @@
+import language.experimental.captureChecking
+
+def id[T](x: T): T = x
+
+trait A:
+  def t(): this.type = id(this)