Don't capture wildcards if in closure or by-name

Author: dwijnand

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4470,6 +4470,8 @@ object Types {
 
     def hasWildcardArg(using Context): Boolean = args.exists(isBounds)
 
+    def hasCaptureConversionArg(using Context): Boolean = args.exists(_.typeSymbol == defn.TypeBox_CAP)
+
     def derivedAppliedType(tycon: Type, args: List[Type])(using Context): Type =
       if ((tycon eq this.tycon) && (args eq this.args)) this
       else tycon.appliedTo(args)

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

@@ -13,6 +13,7 @@ import Decorators._
 import Uniques._
 import inlines.Inlines
 import config.Printers.typr
+import ErrorReporting.*
 import util.SourceFile
 import TypeComparer.necessarySubType
 
@@ -492,7 +493,14 @@ object ProtoTypes {
       val targ = cacheTypedArg(arg,
         typer.typedUnadapted(_, wideFormal, locked)(using argCtx),
         force = true)
-      typer.adapt(targ, wideFormal, locked)
+      val targ1 = typer.adapt(targ, wideFormal, locked)
+      if wideFormal eq formal then targ1
+      else targ1.tpe match
+        case tp: AppliedType if tp.hasCaptureConversionArg =>
+          errorTree(targ1,
+            em"""argument for by-name parameter contains capture conversion skolem types:
+                |$tp""")
+        case _ => targ1
     }
 
     /** The type of the argument `arg`, or `NoType` if `arg` has not been typed before

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

@@ -1568,6 +1568,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 else if ((tree.tpt `eq` untpd.ContextualEmptyTree) && mt.paramNames.isEmpty)
                   // Note implicitness of function in target type since there are no method parameters that indicate it.
                   TypeTree(defn.FunctionOf(Nil, mt.resType, isContextual = true, isErased = false))
+                else if mt.resType.match { case tp: AppliedType => tp.hasCaptureConversionArg case _ => false } then
+                  errorTree(tree,
+                    em"""cannot turn method type $mt into closure
+                        |because it has capture conversion skolem types""")
                 else
                   EmptyTree
             }

tests/neg/t9419.scala

@@ -0,0 +1,22 @@
+trait Magic[S]:
+  def init: S
+  def step(s: S): String
+
+object IntMagic extends Magic[Int]:
+  def init = 0
+  def step(s: Int): String = (s - 1).toString
+
+object StrMagic extends Magic[String]:
+  def init = "hi"
+  def step(s: String): String = s.reverse
+
+object Main:
+  def onestep[T](m: () => Magic[T]): String = m().step(m().init)
+  def unostep[T](m:    => Magic[T]): String = m.step(m.init)
+
+  val iter: Iterator[Magic[?]] = Iterator(IntMagic, StrMagic)
+
+  // was: class java.lang.String cannot be cast to class java.lang.Integer
+  def main(args: Array[String]): Unit =
+    onestep(() => iter.next()) // error
+    unostep(iter.next())       // error