Generate toString only for synthetic companions of case classes

Don't generate a toString method if the companion is explicitly given.
This matches Scala 2's behavior.

Fixes #16879

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -541,7 +541,11 @@ class Namer { typer: Typer =>
           res = cpy.TypeDef(modCls)(
             rhs = cpy.Template(modTempl)(
               derived = if (fromTempl.derived.nonEmpty) fromTempl.derived else modTempl.derived,
-              body = fromTempl.body ++ modTempl.body))
+              body = fromTempl.body.filter {
+                  case stat: DefDef => stat.name != nme.toString_
+                    // toString should only be generated if explicit companion is missing
+                  case _ => true
+                } ++ modTempl.body))
           if (fromTempl.derived.nonEmpty) {
             if (modTempl.derived.nonEmpty)
               report.error(em"a class and its companion cannot both have `derives` clauses", mdef.srcPos)

tests/run/i16879.scala

@@ -0,0 +1,16 @@
+trait Companion:
+  final override def toString: String = "Companion"
+
+case class Example(value: Int)
+object Example extends Companion
+
+case class C()
+object C:
+  override def toString = "CC"
+
+case class D()
+
+@main def Test =
+  assert(Example.toString == "Companion")
+  assert(C.toString == "CC")
+  assert(D.toString == "D")