fix: look at the underlying of a TermRef for a getter (#24565)

If the type of a `val` is a `TermRef`, generating the generic signature
based on the underlying type will produce the type
`scala.Function0<underlying>`. The reason behind this is that during the
`getters` phase, the same symbol will now refer to the getter where the
type will be `=> <underlying>`. Since the `TermRef` originally intended
to capture the underlying type of a `val`, we recover that information
by directly checking the resultType of the getter.

Closes #24553

Author: hamzaremmal

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

@@ -246,6 +246,16 @@ object GenericSignatures {
             jsig(erasedUnderlying, toplevel = toplevel, unboxedVCs = unboxedVCs)
           else typeParamSig(ref.paramName.lastPart)
 
+        case ref: TermRef if ref.symbol.isGetter =>
+          // If the type of a val is a TermRef to another val, generating the generic signature
+          // based on the underlying type will produce the type `scala.Function0<underlying>`
+          // The reason behind this is that during the `getters` phase, the same symbol will now
+          // refer to the getter where the type will be now `=> <underlying>`.
+          // Since the TermRef originally intended to capture the underlying type of a `val`,
+          // we recover that information by directly checking the resultType of the getter.
+          // See `tests/run/i24553.scala` for an example
+          jsig(ref.info.resultType, toplevel = toplevel, unboxedVCs = unboxedVCs)
+
         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 

tests/run/i24553.check

@@ -0,0 +1,11 @@
+public boolean java.lang.Object.equals(java.lang.Object)
+public final native java.lang.Class<?> java.lang.Object.getClass()
+public native int java.lang.Object.hashCode()
+public int Foo.hello()
+public final native void java.lang.Object.notify()
+public final native void java.lang.Object.notifyAll()
+public java.lang.String java.lang.Object.toString()
+public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
+public final void java.lang.Object.wait() throws java.lang.InterruptedException
+public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
+public int Foo.x()

tests/run/i24553.scala

@@ -0,0 +1,8 @@
+// scalajs: --skip
+class Foo:
+    val hello = 1337
+    val x: hello.type = ???
+
+@main def Test =
+    val mtds = classOf[Foo].getMethods().sortBy(_.getName())
+    for mtd <- mtds do println(mtd.toGenericString())