Don't capture wildcards if in closure or by-name (#16732)

Fixes scala/bug#9419

Author: odersky

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

@@ -542,6 +542,10 @@ object Inferencing {
     case tp: AnnotatedType => tp.derivedAnnotatedType(captureWildcards(tp.parent), tp.annot)
     case _ => tp
   }
+
+  def hasCaptureConversionArg(tp: Type)(using Context): Boolean = tp match
+    case tp: AppliedType => tp.args.exists(_.typeSymbol == defn.TypeBox_CAP)
+    case _ => false
 }
 
 trait Inferencing { this: Typer =>

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

@@ -13,6 +13,8 @@ import Decorators._
 import Uniques._
 import inlines.Inlines
 import config.Printers.typr
+import Inferencing.*
+import ErrorReporting.*
 import util.SourceFile
 import TypeComparer.necessarySubType
 
@@ -492,7 +494,23 @@ 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 checkNoWildcardCaptureForCBN(targ1)
+    }
+
+    def checkNoWildcardCaptureForCBN(targ1: Tree)(using Context): Tree = {
+      if hasCaptureConversionArg(targ1.tpe) then
+        stripCast(targ1).tpe match
+          case tp: AppliedType if tp.hasWildcardArg =>
+            errorTree(targ1,
+              em"""argument for by-name parameter is not a value
+                  |and contains wildcard arguments: $tp
+                  |
+                  |Assign it to a val and pass that instead.
+                  |""")
+          case _ => targ1
+      else targ1
     }
 
     /** The type of the argument `arg`, or `NoType` if `arg` has not been typed before

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

@@ -1570,6 +1570,10 @@ 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 hasCaptureConversionArg(mt.resType) 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 +1,24 @@
+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.tabulate(Int.MaxValue)(i => if i % 2 == 0 then IntMagic else 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
+    val m = iter.next()
+    unostep(m)                 // ok, because m is a value

tests/pos/t9419.jackson.scala

@@ -0,0 +1,20 @@
+// from failure in the community project
+// jackson-module-scala
+// in ScalaAnnotationIntrospectorModule.scala:139:12
+
+import scala.language.implicitConversions
+
+trait EnrichedType[X]:
+  def value: X
+
+trait ClassW extends EnrichedType[Class[_]]:
+  def extendsScalaClass = false
+
+class Test:
+  implicit def mkClassW(c: => Class[_]): ClassW = new ClassW:
+    lazy val value = c
+
+  def test1(c1: Class[_]) = c1.extendsScalaClass           // ok: c1 is a value
+  def test2(c2: Class[_]) = mkClassW(c2).extendsScalaClass // ok: c2 is a value
+  // c1 in test1 goes throw adapting to find the extension method and gains the wildcard capture cast then
+  // c2 in test2 goes straight to typedArg, as it's already an arg, so it never gets wildcard captured