Drop retains annotations in inferred type trees

Author: Linyxus

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

@@ -445,6 +445,16 @@ extension (tp: AnnotatedType)
     case ann: CaptureAnnotation => ann.boxed
     case _ => false
 
+class CleanupRetains(using Context) extends TypeMap:
+  def apply(tp: Type): Type = cleanupRetains(tp, this)
+
+/** Drop retains annotations in the type. */
+def cleanupRetains(tp: Type, theMap: CleanupRetains | Null = null)(using Context): Type =
+  def mapOver = (if theMap != null then theMap else new CleanupRetains).mapOver(tp)
+  tp match
+    case RetainingType(tp, _) => tp
+    case _ => mapOver
+
 /** An extractor for `caps.reachCapability(ref)`, which is used to express a reach
  *  capability as a tree in a @retains annotation.
  */

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -48,7 +48,7 @@ import staging.StagingLevel
 import reporting.*
 import Nullables.*
 import NullOpsDecorator.*
-import cc.{CheckCaptures, isRetainsLike}
+import cc.{CheckCaptures, isRetainsLike, cleanupRetains}
 import config.Config
 import config.MigrationVersion
 
@@ -2187,7 +2187,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             errorTree(tree, em"Something's wrong: missing original symbol for type tree")
         }
       case _ =>
-        completeTypeTree(InferredTypeTree(), pt, tree)
+        val pt1 = cleanupRetains(pt)
+          // Cleans up retains annotations in inferred type trees. This is needed because
+          // during the typer, it is infeasible to correctly infer the capture sets in most
+          // cases, resulting ill-formed capture sets that could crash the pickler later on.
+          // See #20035.
+        completeTypeTree(InferredTypeTree(), pt1, tree)
 
   def typedInLambdaTypeTree(tree: untpd.InLambdaTypeTree, pt: Type)(using Context): Tree =
     val tp =

tests/pos-custom-args/captures/tablediff.scala

@@ -0,0 +1,11 @@
+import language.experimental.captureChecking
+
+trait Seq[+A]:
+  def zipAll[A1 >: A, B](that: Seq[B]^, thisElem: A1, thatElem: B): Seq[(A1, B)]^{this, that}
+  def map[B](f: A => B): Seq[B]^{this, f}
+
+def zipAllOption[X](left: Seq[X], right: Seq[X]) =
+  left.map(Option(_)).zipAll(right.map(Option(_)), None, None)
+
+def fillRow[T](headRow: Seq[T], tailRow: Seq[T]) =
+  val paddedZip = zipAllOption(headRow, tailRow)