Fix #5720: don't output X <: Long in Java generic signatures

In generic signature generator `jsig`, flag `primitiveOK` remembers if `jsig` is
allowed to output primitive types or not; for instance, upper bounds can't
contain primitive types. When transforming higher-kinded upper bounds `[X] =>
T`, `jsig` correctly recurs on `T`, but must remember flag `primitiveOK`.

The status of `toplevel` should not change either (since it tracks if are still
at the toplevel of the output signature, or instead we have started producing
output), so passing it seems safest. Ideally we should test it at least on
higher types whose body is a suitable `ClassInfo`; with polymorphic function
types, we should also test `PolyType` bodies.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -283,7 +283,7 @@ object GenericSignatures {
           jsig(atp, toplevel, primitiveOK)
 
         case hktl: HKTypeLambda =>
-          jsig(hktl.finalResultType)
+          jsig(hktl.finalResultType, toplevel, primitiveOK)
 
         case _ =>
           val etp = erasure(tp)

tests/pos/i5720.scala

@@ -0,0 +1,5 @@
+class A[B[_] <: Long]
+object Main {
+  type C = A[[x] => Long]
+  type D = A[[x] => Nothing]
+}