Fix the non-miniphase tree traverser (#16684)

Fixes #16678. The issue is related to the second tree traverser (needed
as miniphase traverser do not cover every cases).

Symbols are added to a set during the traversal of their own definition
to avoid recording recursive. In the second tree traverser this symbol
is never removed, so the following usage are never cached, resulting in
false positive.

@szymon-rd

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

@@ -204,12 +204,15 @@ class CheckUnused extends MiniPhase:
         case t:tpd.ValDef =>
           prepareForValDef(t)
           traverseChildren(tree)(using newCtx)
+          transformValDef(t)
         case t:tpd.DefDef =>
           prepareForDefDef(t)
           traverseChildren(tree)(using newCtx)
+          transformDefDef(t)
         case t:tpd.TypeDef =>
           prepareForTypeDef(t)
           traverseChildren(tree)(using newCtx)
+          transformTypeDef(t)
         case t: tpd.Bind =>
           prepareForBind(t)
           traverseChildren(tree)(using newCtx)
@@ -332,7 +335,7 @@ object CheckUnused:
      * The optional name will be used to target the right import
      * as the same element can be imported with different renaming
      */
-    def registerUsed(sym: Symbol, name: Option[Name])(using Context): Unit =
+    def registerUsed(sym: Symbol, name: Option[Name])(using Context): Unit =      
       if !isConstructorOfSynth(sym) && !doNotRegister(sym) then
         if sym.isConstructor && sym.exists then
           registerUsed(sym.owner, None) // constructor are "implicitly" imported with the class
@@ -368,7 +371,7 @@ object CheckUnused:
             implicitParamInScope += memDef
           else
             explicitParamInScope += memDef
-        else if currScopeType.top == ScopeType.Local then
+        else if currScopeType.top == ScopeType.Local then 
           localDefInScope += memDef
         else if memDef.shouldReportPrivateDef then
           privateDefInScope += memDef

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

@@ -75,6 +75,16 @@ package foo.test.companionprivate:
     private def b = c // OK
     def c = List(1,2,3) // OK
 
+package foo.test.i16678:
+  def foo(func: Int => String, value: Int): String = func(value) // OK
+
+  def run = 
+    println(foo(number => number.toString, value = 5)) // OK
+    println(foo(number => "<number>", value = 5)) // error
+    println(foo(func = number => "<number>", value = 5)) // error
+    println(foo(func = number => number.toString, value = 5)) // OK
+    println(foo(func = _.toString, value = 5)) // OK
+
 package foo.test.possibleclasses:
   case class AllCaseClass(
     k: Int, // OK
@@ -120,4 +130,4 @@ package foo.test.from.i16675:
   case class PositiveNumber private (i: Int) // OK
   object PositiveNumber:
     def make(i: Int): Option[PositiveNumber] = //OK 
-      Option.when(i >= 0)(PositiveNumber(i)) // OK
+      Option.when(i >= 0)(PositiveNumber(i)) // OK