Allow wildcard capture of by-name values

Author: dwijnand

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

@@ -497,9 +497,15 @@ object ProtoTypes {
       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""")
+          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
         case _ => targ1
     }
 

tests/neg/t9419.scala

@@ -14,9 +14,11 @@ 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)
+  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