WUnused: Fix unused warnining in synthetic symbols (#17020)

Resolves #16925 #16926

Author: Kordyjan

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

@@ -7,13 +7,13 @@ import dotty.tools.dotc.ast.untpd.ImportSelector
 import dotty.tools.dotc.config.ScalaSettings
 import dotty.tools.dotc.core.Contexts.*
 import dotty.tools.dotc.core.Decorators.{em, i}
-import dotty.tools.dotc.core.Flags._
+import dotty.tools.dotc.core.Flags.*
 import dotty.tools.dotc.core.Phases.Phase
 import dotty.tools.dotc.core.StdNames
 import dotty.tools.dotc.report
 import dotty.tools.dotc.reporting.Message
 import dotty.tools.dotc.typer.ImportInfo
-import dotty.tools.dotc.util.Property
+import dotty.tools.dotc.util.{Property, SrcPos}
 import dotty.tools.dotc.core.Mode
 import dotty.tools.dotc.core.Types.TypeTraverser
 import dotty.tools.dotc.core.Types.Type
@@ -302,6 +302,7 @@ object CheckUnused:
      * See the `isAccessibleAsIdent` extension method below in the file
      */
     private val usedInScope = MutStack(MutSet[(Symbol,Boolean, Option[Name])]())
+    private val usedInPosition = MutSet[(SrcPos, Name)]()
     /* unused import collected during traversal */
     private val unusedImport = MutSet[ImportSelector]()
 
@@ -351,6 +352,7 @@ object CheckUnused:
           usedInScope.top += ((sym, sym.isAccessibleAsIdent, name))
           usedInScope.top += ((sym.companionModule, sym.isAccessibleAsIdent, name))
           usedInScope.top += ((sym.companionClass, sym.isAccessibleAsIdent, name))
+          name.map(n => usedInPosition += ((sym.sourcePos, n)))
 
     /** Register a symbol that should be ignored */
     def addIgnoredUsage(sym: Symbol)(using Context): Unit =
@@ -455,6 +457,7 @@ object CheckUnused:
         if ctx.settings.WunusedHas.locals then
           localDefInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => usedInPosition.exists { case (pos, name) => d.span.contains(pos.span) && name == d.symbol.name})
             .map(d => d.namePos -> WarnTypes.LocalDefs).toList
         else
           Nil
@@ -483,6 +486,7 @@ object CheckUnused:
         if ctx.settings.WunusedHas.patvars then
           patVarsInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => usedInPosition.exists { case (pos, name) => d.span.contains(pos.span) && name == d.symbol.name})
             .map(d => d.namePos -> WarnTypes.PatVars).toList
         else
           Nil

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

@@ -194,3 +194,17 @@ package foo.test.i16877:
 
   @ExampleAnnotation(new HashMap()) // OK
   class Test //OK
+
+package foo.test.i16926:
+  def hello(): Unit =
+    for {
+      i <- (0 to 10).toList
+      (a, b) = "hello" -> "world" // OK
+    } yield println(s"$a $b")
+
+package foo.test.i16925:
+  def hello =
+    for {
+      i <- 1 to 2 if true
+      _ = println(i) // OK
+    } yield ()