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.check

@@ -5,10 +5,3 @@
  | Required: A
  |
  | longer explanation available when compiling with `-explain`
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23207.scala:16:11 ---------------------------------------
-16 | b.getBox.x // error
- | ^^^^^^^^^^
- | Found: (Box[(b : B^{io})^{b}]#x : (b : B^{io})^{b})
- | Required: A^?
- |
- | longer explanation available when compiling with `-explain`

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)