Fix -Wunused:import enum

- Some enum generates an import clause for its cases
- Update some test suits

Author: PaulCoral

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

@@ -331,6 +331,8 @@ object CheckUnused:
           registerUsed(sym.owner, None) // constructor are "implicitly" imported with the class
         else
           usedInScope.top += ((sym, sym.isAccessibleAsIdent, name))
+          usedInScope.top += ((sym.companionModule, sym.isAccessibleAsIdent, name))
+          usedInScope.top += ((sym.companionClass, sym.isAccessibleAsIdent, name))
 
     /** Register a symbol that should be ignored */
     def addIgnoredUsage(sym: Symbol)(using Context): Unit =
@@ -347,7 +349,7 @@ object CheckUnused:
 
     /** Register an import */
     def registerImport(imp: tpd.Import)(using Context): Unit =
-      if !tpd.languageImport(imp.expr).nonEmpty then
+      if !tpd.languageImport(imp.expr).nonEmpty && !imp.isGeneratedByEnum then
         impInScope.top += imp
         unusedImport ++= imp.selectors.filter { s =>
           !shouldSelectorBeReported(imp, s) && !isImportExclusion(s)
@@ -591,6 +593,10 @@ object CheckUnused:
         !isSyntheticMainParam(sym)  &&
         !sym.shouldNotReportParamOwner
 
+    extension (imp: tpd.Import)
+      /** Enum generate an import for its cases (but outside them), which should be ignored */
+      def isGeneratedByEnum(using Context): Boolean =
+        imp.symbol.exists && imp.symbol.owner.is(Flags.Enum, butNot = Flags.Case)
 
     extension (thisName: Name)
       private def isWildcard: Boolean =

tests/neg-custom-args/fatal-warnings/i15503a.scala

@@ -250,4 +250,9 @@ package foo.testing.rename.imports:
 package foo.testing.imports.precedence:
   import scala.collection.immutable.{BitSet => _, _} // error
   import scala.collection.immutable.BitSet // OK
-  def t = BitSet.empty
+  def t = BitSet.empty
+
+package foo.test.enums:
+  enum A: // OK
+    case B extends A // OK
+    case C extends A // OK

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

@@ -65,3 +65,12 @@ package foo.test.scala.annotation:
     @unused private def c = 3 // OK
 
     def other = b
+
+package foo.test.companionprivate:
+  class A:
+    import A.b // OK
+    def a = b // OK
+
+  object A:
+    private def b = c // OK
+    def c = List(1,2,3) // OK