Traverse annotations instead of just registering, fixes #16877 (#16956)

@szymon-rd  Fixes #16877 
- Traverse the tree of annotations
- Update test suits

The issue is that the annotation was registered but not traversed.

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

@@ -28,6 +28,7 @@ import dotty.tools.dotc.core.Types.ConstantType
 import dotty.tools.dotc.core.NameKinds.WildcardParamName
 import dotty.tools.dotc.core.Types.TermRef
 import dotty.tools.dotc.core.Types.NameFilter
+import dotty.tools.dotc.core.Symbols.Symbol
 
 
 
@@ -102,6 +103,7 @@ class CheckUnused extends MiniPhase:
   override def prepareForValDef(tree: tpd.ValDef)(using Context): Context =
     unusedDataApply{ud =>
       // do not register the ValDef generated for `object`
+      traverseAnnotations(tree.symbol)
       if !tree.symbol.is(Module) then
         ud.registerDef(tree)
       ud.addIgnoredUsage(tree.symbol)
@@ -111,18 +113,21 @@ class CheckUnused extends MiniPhase:
     unusedDataApply{ ud =>
       import ud.registerTrivial
       tree.registerTrivial
+      traverseAnnotations(tree.symbol)
       ud.registerDef(tree)
       ud.addIgnoredUsage(tree.symbol)
     }
 
   override def prepareForTypeDef(tree: tpd.TypeDef)(using Context): Context =
     unusedDataApply{ ud =>
       if !tree.symbol.is(Param) then // Ignore type parameter (as Scala 2)
+        traverseAnnotations(tree.symbol)
         ud.registerDef(tree)
         ud.addIgnoredUsage(tree.symbol)
     }
 
   override def prepareForBind(tree: tpd.Bind)(using Context): Context =
+    traverseAnnotations(tree.symbol)
     unusedDataApply(_.registerPatVar(tree))
 
   override def prepareForTypeTree(tree: tpd.TypeTree)(using Context): Context =
@@ -237,6 +242,10 @@ class CheckUnused extends MiniPhase:
         case _ =>
           traverseChildren(tp)
 
+  /** This traverse the annotations of the symbol */
+  private def traverseAnnotations(sym: Symbol)(using Context): Unit =
+    sym.denot.annotations.foreach(annot => traverser.traverse(annot.tree))
+
   /** Do the actual reporting given the result of the anaylsis */
   private def reportUnused(res: UnusedData.UnusedResult)(using Context): Unit =
     import CheckUnused.WarnTypes
@@ -279,7 +288,6 @@ object CheckUnused:
   private class UnusedData:
     import dotty.tools.dotc.transform.CheckUnused.UnusedData.UnusedResult
     import collection.mutable.{Set => MutSet, Map => MutMap, Stack => MutStack}
-    import dotty.tools.dotc.core.Symbols.Symbol
     import UnusedData.ScopeType
 
     /** The current scope during the tree traversal */
@@ -329,11 +337,6 @@ object CheckUnused:
       execInNewScope
       popScope()
 
-    /** Register all annotations of this symbol's denotation */
-    def registerUsedAnnotation(sym: Symbol)(using Context): Unit =
-      val annotSym = sym.denot.annotations.map(_.symbol)
-      annotSym.foreach(s => registerUsed(s, None))
-
     /**
      * Register a found (used) symbol along with its name
      *
@@ -368,8 +371,6 @@ object CheckUnused:
 
     /** Register (or not) some `val` or `def` according to the context, scope and flags */
     def registerDef(memDef: tpd.MemberDef)(using Context): Unit =
-      // register the annotations for usage
-      registerUsedAnnotation(memDef.symbol)
       if memDef.isValidMemberDef then
         if memDef.isValidParam then
           if memDef.symbol.isOneOf(GivenOrImplicit) then
@@ -383,7 +384,6 @@ object CheckUnused:
 
     /** Register pattern variable */
     def registerPatVar(patvar: tpd.Bind)(using Context): Unit =
-      registerUsedAnnotation(patvar.symbol)
       if !patvar.symbol.isUnusedAnnot then
         patVarsInScope += patvar
 

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

@@ -185,3 +185,12 @@ package foo.test.i16822:
     val x = ExampleEnum.List // OK
     println(x) // OK
   }
+
+package foo.test.i16877:
+  import scala.collection.immutable.HashMap // OK
+  import scala.annotation.StaticAnnotation // OK
+
+  class ExampleAnnotation(val a: Object) extends StaticAnnotation // OK
+
+  @ExampleAnnotation(new HashMap()) // OK
+  class Test //OK