fix constructors Java generic signatures

Author: xuwei-k

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

@@ -291,7 +291,7 @@ object GenericSignatures {
 
         case PolyType(tparams, mtpe: MethodType) =>
           assert(tparams.nonEmpty)
-          if (toplevel) polyParamSig(tparams)
+          if (toplevel && !sym0.isConstructor) polyParamSig(tparams)
           jsig(mtpe)
 
         // Nullary polymorphic method

tests/generic-java-signatures/i10834.check

@@ -0,0 +1,6 @@
+0
+
+public A1(X)
+0
+
+public A2(java.lang.Object)

tests/generic-java-signatures/i10834.scala

@@ -0,0 +1,16 @@
+class A1[X](val x: X)
+class A2[F[_]](val x: F[String])
+
+object Test {
+  def test(clazz: Class[?]): Unit = {
+    val List(constructor) = clazz.getConstructors().toList
+    println(constructor.getTypeParameters().length)
+    println(constructor.getTypeParameters().mkString(", "))
+    println(constructor.toGenericString())
+  }
+
+  def main(args: Array[String]): Unit = {
+    test(classOf[A1[?]])
+    test(classOf[A2[?]])
+  }
+}