Avoid spurious dealiasing in alignDependentFunction

This would become a problem when adding fluid capture sets to
FromJavaObject types. And it anyway gives better error messages.

Author: odersky

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

@@ -642,8 +642,10 @@ class CheckCaptures extends Recheck, SymTransformer:
     /** Turn `expected` into a dependent function when `actual` is dependent. */
     private def alignDependentFunction(expected: Type, actual: Type)(using Context): Type =
       def recur(expected: Type): Type = expected.dealias match
-        case expected @ CapturingType(eparent, refs) =>
-          CapturingType(recur(eparent), refs, boxed = expected.isBoxed)
+        case expected0 @ CapturingType(eparent, refs) =>
+          val eparent1 = recur(eparent)
+          if eparent1 eq eparent then expected
+          else CapturingType(eparent1, refs, boxed = expected0.isBoxed)
         case expected @ defn.FunctionOf(args, resultType, isContextual)
           if defn.isNonRefinedFunction(expected) && defn.isFunctionType(actual) && !defn.isNonRefinedFunction(actual) =>
           val expected1 = toDepFun(args, resultType, isContextual)

tests/neg-custom-args/captures/i15772.check

@@ -16,7 +16,7 @@
 33 |  val boxed2 : Observe[C]^ = box2(c) // error
    |                             ^^^^^^^
    |                             Found:    (C{val arg: C^}^ => Unit) ->? Unit
-   |                             Required: (C => Unit) => Unit
+   |                             Required: Observe[C]^
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:44:2 ----------------------------------------