Test for contravariantly used class fields

As discussed in the CC meeting on 21 Jan 2022

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -208,6 +208,7 @@ class TreePickler(pickler: TastyPickler) {
         writeByte(if (tpe.isType) TYPEREFdirect else TERMREFdirect)
         if Config.checkLevels && !symRefs.contains(sym) && !sym.isPatternBound && !sym.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
           report.error(i"pickling reference to as yet undefined $tpe with symbol ${sym}", sym.srcPos)
+            // todo: find out why this happens for pos-customargs/captures/capt2
         pickleSymRef(sym)
       }
       else tpe.designator match {

tests/neg-custom-args/captures/class-contra.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/class-contra.scala:12:39 ---------------------------------
+12 |  def fun(x: K{val f: {a} T}) = x.setf(a) // error
+   |                                       ^
+   |                                       Found:    (a : {x, y} T)
+   |                                       Required: T
+
+longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/class-contra.scala

@@ -0,0 +1,13 @@
+
+class C
+type Cap = {*} C
+
+class K(val f: {*} T):
+  def setf(x: {f} T) = ???
+
+class T
+
+def test(x: Cap, y: Cap) =
+  val a: {x, y} T = ???
+  def fun(x: K{val f: {a} T}) = x.setf(a) // error
+  ()