Backport "Fix WUnused false positive in for" (#17278)

Backports #17176

Author: Kordyjan

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

@@ -358,7 +358,8 @@ object CheckUnused:
           usedInScope.top += ((sym, sym.isAccessibleAsIdent, name, isDerived))
           usedInScope.top += ((sym.companionModule, sym.isAccessibleAsIdent, name, isDerived))
           usedInScope.top += ((sym.companionClass, sym.isAccessibleAsIdent, name, isDerived))
-          name.map(n => usedInPosition += ((sym.sourcePos, n)))
+          if sym.sourcePos.exists then
+            name.map(n => usedInPosition += ((sym.sourcePos, n)))
 
     /** Register a symbol that should be ignored */
     def addIgnoredUsage(sym: Symbol)(using Context): Unit =
@@ -473,6 +474,7 @@ 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/i15503-scala2/scala2-t11681.scala

@@ -60,7 +60,7 @@ class Revaluing(u: Int) { def f = u } // OK
 
 case class CaseyKasem(k: Int)        // OK
 
-case class CaseyAtTheBat(k: Int)(s: String)        // error
+case class CaseyAtTheBat(k: Int)(s: String)        // ok
 
 trait Ignorance {
   def f(readResolve: Int) = answer           // error

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

@@ -90,7 +90,7 @@ package foo.test.possibleclasses:
     k: Int, // OK
     private val y: Int // OK /* Kept as it can be taken from pattern */
   )(
-    s: Int, // error /* But not these */
+    s: Int,
     val t: Int, // OK
     private val z: Int // error
   )
@@ -131,7 +131,7 @@ package foo.test.possibleclasses.withvar:
     k: Int, // OK
     private var y: Int // OK /* Kept as it can be taken from pattern */
   )(
-    s: Int, // error /* But not these */
+    s: Int,
     var t: Int, // OK
     private var z: Int // error
   )
@@ -300,4 +300,4 @@ package foo.test.i17117:
 
       val test = t1.test
     }
-  }
+  }