Nowarn extension taking params

[Cherry-picked ec9a060]

Author: som-snytt

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

@@ -582,11 +582,11 @@ object CheckUnused:
                   warnAt(pos)(UnusedSymbol.privateMembers)
         else if ctx.settings.WunusedHas.explicits
           && !sym.is(Synthetic) // param to setter is unused bc there is no field yet
-          && !(sym.owner.is(ExtensionMethod) && {
-            m.paramSymss.dropWhile(_.exists(_.isTypeParam)) match
-            case (h :: Nil) :: Nil => h == sym // param is the extended receiver
+          && !(sym.owner.is(ExtensionMethod) &&
+            m.paramSymss.dropWhile(_.exists(_.isTypeParam)).match
+            case (h :: Nil) :: _ => h == sym // param is the extended receiver
             case _ => false
-          })
+          )
           && !sym.name.isInstanceOf[DerivedName]
           && !ctx.platform.isMainMethod(m)
         then

tests/warn/i23349.scala

@@ -0,0 +1,15 @@
+//> using options -Wunused:explicits
+
+// An external class that doesn't get its own `copy` method.
+class Foo(val a: String, val b: Int)
+
+//
+// Example 1: add `copy` method via an extension method.
+//
+extension (self: Foo)
+  def copy(a: String = self.a, b: Int = self.b): Foo = Foo(a, b) // nowarn
+
+//
+// Example 2: implement `copyFoo` with parameter groups.
+//
+def copyFoo(foo: Foo)(a: String = foo.a, b: Int = foo.b): Foo = Foo(a, b) // warn