Avoid calling addOuterRefs when actual type is box-adapted

Author: Linyxus

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -847,9 +847,13 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  where local capture roots are instantiated to root variables.
      */
     override def checkConformsExpr(actual: Type, expected: Type, tree: Tree, addenda: Addenda)(using Context): Type =
-      val expected1 = alignDependentFunction(addOuterRefs(expected, actual), actual.stripCapturing)
+      var expected1 = alignDependentFunction(expected, actual.stripCapturing)
       val actualBoxed = adaptBoxed(actual, expected1, tree.srcPos)
       //println(i"check conforms $actualBoxed <<< $expected1")
+
+      if actualBoxed eq actual then
+        // Only `addOuterRefs` when there is no box adaptation
+        expected1 = addOuterRefs(expected1, actual)
       if isCompatible(actualBoxed, expected1) then
         if debugSuccesses then tree match
             case Ident(_) =>

tests/neg-custom-args/captures/cc-selftype-unsound.scala

@@ -0,0 +1,15 @@
+import language.experimental.captureChecking
+trait Logger
+case class Boxed[T](unbox: T)
+
+// a horrible function, but it typechecks
+def magic(l: Logger^): Logger =
+  class Foo:
+    def foo: Boxed[Logger^{this}] =
+      // for the following line to typecheck
+      //   the capture checker assumes {l} <: {this}
+      Boxed[Logger^{this}](l)  // error
+  val x = new Foo
+  val y = x.foo.unbox  // y: Logger^{x}
+  val z: Logger = y  // now the capability becomes pure
+  z