Fix WUnused false positive in for

Author: szymon-rd

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

@@ -473,6 +473,7 @@ object CheckUnused:
         if ctx.settings.WunusedHas.explicits then
           explicitParamInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => usedInPosition.exists { case (pos, name) => d.span.contains(pos.span) && name == d.symbol.name})
             .filterNot(d => containsSyntheticSuffix(d.symbol))
             .map(d => d.namePos -> WarnTypes.ExplicitParams).toList
         else

tests/neg-custom-args/fatal-warnings/i15503i.scala

@@ -300,4 +300,4 @@ package foo.test.i17117:
 
       val test = t1.test
     }
-  }
+  }