Generate toString only for synthetic companions of case classes (#16890)

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")

tests/semanticdb/metac.expect

@@ -467,7 +467,7 @@ Schema => SemanticDB v4
 Uri => Classes.scala
 Text => empty
 Language => Scala
-Symbols => 109 entries
+Symbols => 108 entries
 Occurrences => 114 entries
 Synthetics => 2 entries
 
@@ -504,10 +504,9 @@ classes/C4#copy$default$1(). => method copy$default$1 => Int @uncheckedVariance
 classes/C4#copy(). => method copy (param x: Int): C4
 classes/C4#copy().(x) => param x: Int
 classes/C4#x. => val method x Int
-classes/C4. => final object C4 extends Object { self: C4.type => +4 decls }
+classes/C4. => final object C4 extends Object { self: C4.type => +3 decls }
 classes/C4.apply(). => method apply (param x: Int): C4
 classes/C4.apply().(x) => param x: Int
-classes/C4.toString(). => method toString => String <: scala/Any#toString().
 classes/C4.unapply(). => method unapply (param x$1: C4): C4
 classes/C4.unapply().(x$1) => param x$1: C4
 classes/C6# => case class C6 extends Object with Product with Serializable { self: C6 => +5 decls }
@@ -4559,7 +4558,7 @@ Schema => SemanticDB v4
 Uri => semanticdb-Types.scala
 Text => empty
 Language => Scala
-Symbols => 144 entries
+Symbols => 143 entries
 Occurrences => 228 entries
 Synthetics => 1 entries
 
@@ -4585,10 +4584,9 @@ types/Foo#copy$default$1(). => method copy$default$1 => "abc" @uncheckedVariance
 types/Foo#copy(). => method copy (param s: "abc"): Foo
 types/Foo#copy().(s) => param s: "abc"
 types/Foo#s. => val method s "abc"
-types/Foo. => final object Foo extends Object { self: Foo.type => +6 decls }
+types/Foo. => final object Foo extends Object { self: Foo.type => +5 decls }
 types/Foo.apply(). => method apply (param s: "abc"): Foo
 types/Foo.apply().(s) => param s: "abc"
-types/Foo.toString(). => method toString => String <: scala/Any#toString().
 types/Foo.unapply(). => method unapply (param x$1: Foo): Foo
 types/Foo.unapply().(x$1) => param x$1: Foo
 types/Foo.x. => val method x "abc" @deprecated