Add better support for trivial methods -Wunused

- Method having a return type of "SingleType" (as named in Scala 2) are
  considered trivial (i.e. Nil, object X, object Y, None, ...)
- Update test suits

Author: PaulCoral

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

@@ -26,6 +26,7 @@ import dotty.tools.dotc.core.Annotations
 import dotty.tools.dotc.core.Definitions
 import dotty.tools.dotc.core.Types.ConstantType
 import dotty.tools.dotc.core.NameKinds.WildcardParamName
+import dotty.tools.dotc.core.Types.TermRef
 
 
 
@@ -569,7 +570,13 @@ object CheckUnused:
         defdef.symbol.is(Deferred) ||
         (rhs match {
           case _: tpd.Literal => true
-          case _ => rhs.tpe.isInstanceOf[ConstantType]
+          case _ => rhs.tpe match
+            case ConstantType(_) => true
+            case tp: TermRef =>
+              // Detect Scala 2 SingleType
+              tp.underlying.classSymbol.is(Flags.Module)
+            case _ =>
+              false
         })
       def registerTrivial(using Context): Unit =
         if defdef.isTrivial then
@@ -590,7 +597,6 @@ object CheckUnused:
       private def isWildcard: Boolean =
         thisName == StdNames.nme.WILDCARD || thisName.is(WildcardParamName)
 
-
   end UnusedData
 
   private object UnusedData:

tests/neg-custom-args/fatal-warnings/i15503e.scala

@@ -24,4 +24,31 @@ package scala2main:
 package scala3main:
   /* This goes around the "trivial method" detection */
   val default_unit = ()
-  @main def hello = println("Hello World") // OK
+  @main def hello = println("Hello World") // OK
+
+package foo.test.lambda.param:
+  val default_val = 1
+  val a = (i: Int) => i // OK
+  val b = (i: Int) => default_val // error
+  val c = (_: Int) => default_val // OK
+
+package foo.test.trivial:
+  /* A twisted test from Scala 2 */
+  class C {
+    def answer: 42 = 42
+    object X
+    def g0(x: Int) = ??? // OK
+    def f0(x: Int) = () // OK
+    def f1(x: Int) = throw new RuntimeException // OK
+    def f2(x: Int) = 42 // OK
+    def f3(x: Int): Option[Int] = None // OK
+    def f4(x: Int) = classOf[Int] // OK
+    def f5(x: Int) = answer + 27 // OK
+    def f6(x: Int) = X // OK
+    def f7(x: Int) = Y // OK
+    def f8(x: Int): List[C] = Nil // OK
+    def f9(x: Int): List[Int] = List(1,2,3,4) // error
+    def foo:Int = 32  // OK
+    def f77(x: Int) = foo // error
+  }
+  object Y