Drop retained elements and keep the annotation

Author: Linyxus

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

@@ -449,7 +449,8 @@ extension (tp: AnnotatedType)
 class CleanupRetains(using Context) extends TypeMap:
   def apply(tp: Type): Type =
     tp match
-      case RetainingType(tp, _) => tp
+      case AnnotatedType(tp, annot) if annot.symbol == defn.RetainsAnnot || annot.symbol == defn.RetainsByNameAnnot =>
+        RetainingType(tp, Nil, byName = annot.symbol == defn.RetainsByNameAnnot)
       case _ => mapOver(tp)
 
 /** An extractor for `caps.reachCapability(ref)`, which is used to express a reach

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

@@ -454,7 +454,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           case _ => false
 
         def signatureChanges =
-          (tree.tpt.hasRememberedType || tree.tpt.isInstanceOf[InferredTypeTree]) && !sym.isConstructor || paramSignatureChanges
+          tree.tpt.hasRememberedType && !sym.isConstructor || paramSignatureChanges
 
         // Replace an existing symbol info with inferred types where capture sets of
         // TypeParamRefs and TermParamRefs put in correspondence by BiTypeMaps with the

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

@@ -9,7 +9,7 @@
    |  Found:    (x$0: Int) ->{cap2} Int
    |  Required: (x$0: Int) -> Int
    |
-   |  Note that the expected type Int -> Int
+   |  Note that the expected type Int ->{} Int
    |  is the previously inferred result type of method test
    |  which is also the type seen in separately compiled sources.
    |  The new inferred type (x$0: Int) ->{cap2} Int