Addapt GenericSignatures to handle unused parameters

Author: nicolasstucki

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

@@ -239,8 +239,8 @@ object GenericSignatures {
           methodResultSig(restpe)
 
         case mtpe: MethodType =>
-          // phantom method parameters do not make it to the bytecode.
-          val params = mtpe.paramInfos.filterNot(_.isPhantom)
+          // unused method parameters do not make it to the bytecode.
+          val params = if (mtpe.isUnusedMethod) Nil else mtpe.paramInfos
           val restpe = mtpe.resultType
           builder.append('(')
           // TODO: Update once we support varargs

tests/generic-java-signatures/unused.check

@@ -0,0 +1,2 @@
+public <U> int MyUnused$.f1()
+U <: java.lang.Object

tests/generic-java-signatures/unused.scala

@@ -0,0 +1,14 @@
+object MyUnused {
+  def f1[U](unused a: Int): Int = 0
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val f1 = MyUnused.getClass.getMethods.find(_.getName.endsWith("f1")).get
+    val tParams = f1.getTypeParameters
+    println(f1.toGenericString)
+    tParams.foreach { tp =>
+      println(tp.getName + " <: " + tp.getBounds.map(_.getTypeName).mkString(", "))
+    }
+  }
+}