improve the generic signatures for singleton types

Author: hamzaremmal

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

@@ -246,6 +246,12 @@ object GenericSignatures {
             jsig(erasedUnderlying, toplevel = toplevel, unboxedVCs = unboxedVCs)
           else typeParamSig(ref.paramName.lastPart)
 
+        case ref: SingletonType =>
+          // Singleton types like `x.type` need to be widened to their underlying type
+          // For example, `def identity[A](x: A): x.type` should have signature 
+          // with return type `A` (not `java.lang.Object`)
+          jsig(ref.underlying, toplevel = toplevel, unboxedVCs = unboxedVCs)
+
         case defn.ArrayOf(elemtp) =>
           if (isGenericArrayElement(elemtp, isScala2 = false))
             jsig1(defn.ObjectType)

tests/run/i24272.check

@@ -0,0 +1 @@
+public <B> B Foo.bar(B)

tests/run/i24272.scala

@@ -0,0 +1,8 @@
+// scalajs: --skip
+
+class Foo:
+  def bar[B](x: B): x.type = x
+
+@main def Test =
+  for mtd <- classOf[Foo].getDeclaredMethods.sortBy(_.getName) do
+    println(mtd.toGenericString)