Warn for synthetic using/givens with wunused

Author: szymon-rd

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

@@ -478,7 +478,7 @@ object CheckUnused:
         if ctx.settings.WunusedHas.implicits then
           implicitParamInScope
             .filterNot(d => d.symbol.usedDefContains)
-            .filterNot(d => containsSyntheticSuffix(d.symbol))
+            .filterNot(d => containsSyntheticSuffix(d.symbol) && !d.rawMods.is(Given))
             .map(d => d.namePos -> WarnTypes.ImplicitParams).toList
         else
           Nil

tests/neg-custom-args/fatal-warnings/i15503f.scala

@@ -5,7 +5,7 @@ val default_int = 1
 
 def f1(a: Int) = a // OK
 def f2(a: Int) = 1 // OK
-def f3(a: Int)(using Int) = a // OK
+def f3(a: Int)(using Int) = a // error
 def f4(a: Int)(using Int) = default_int // error
 def f6(a: Int)(using Int) = summon[Int] // OK
 def f7(a: Int)(using Int) = summon[Int] + a // OK