Handle @companionClass and @companionMethod meta-annotations

Fixes #17002

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1028,6 +1028,8 @@ class Definitions {
   @tu lazy val GetterMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.getter")
   @tu lazy val ParamMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.param")
   @tu lazy val SetterMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.setter")
+  @tu lazy val CompanionClassMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.companionClass")
+  @tu lazy val CompanionMethodMetaAnnot: ClassSymbol = requiredClass("scala.annotation.meta.companionMethod")
   @tu lazy val ShowAsInfixAnnot: ClassSymbol = requiredClass("scala.annotation.showAsInfix")
   @tu lazy val FunctionalInterfaceAnnot: ClassSymbol = requiredClass("java.lang.FunctionalInterface")
   @tu lazy val TargetNameAnnot: ClassSymbol = requiredClass("scala.annotation.targetName")
@@ -1041,7 +1043,7 @@ class Definitions {
 
   // A list of meta-annotations that are relevant for fields and accessors
   @tu lazy val NonBeanMetaAnnots: Set[Symbol] =
-    Set(FieldMetaAnnot, GetterMetaAnnot, ParamMetaAnnot, SetterMetaAnnot)
+    Set(FieldMetaAnnot, GetterMetaAnnot, ParamMetaAnnot, SetterMetaAnnot, CompanionClassMetaAnnot, CompanionMethodMetaAnnot)
   @tu lazy val MetaAnnots: Set[Symbol] =
     NonBeanMetaAnnots + BeanGetterMetaAnnot + BeanSetterMetaAnnot
 

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -255,10 +255,13 @@ object SymDenotations {
     def annotationsCarrying(meta: Set[Symbol], orNoneOf: Set[Symbol] = Set.empty)(using Context): List[Annotation] =
       annotations.filterConserve(_.hasOneOfMetaAnnotation(meta, orNoneOf = orNoneOf))
 
-    def copyAndKeepAnnotationsCarrying(phase: DenotTransformer, meta: Set[Symbol], orNoneOf: Set[Symbol] = Set.empty)(using Context): Unit =
-      if annotations.nonEmpty then
+    def keepAnnotationsCarrying(phase: DenotTransformer, meta: Set[Symbol], orNoneOf: Set[Symbol] = Set.empty)(using Context): Unit =
+      updateAnnotationsAfter(phase, annotationsCarrying(meta, orNoneOf = orNoneOf))
+
+    def updateAnnotationsAfter(phase: DenotTransformer, annots: List[Annotation])(using Context): Unit =
+      if annots ne annotations then
         val cpy = copySymDenotation()
-        cpy.annotations = annotationsCarrying(meta, orNoneOf = orNoneOf)
+        cpy.annotations = annots
         cpy.installAfter(phase)
 
     /** Optionally, the annotation matching the given class symbol */

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

@@ -167,7 +167,7 @@ class Memoize extends MiniPhase with IdentityDenotTransformer { thisPhase =>
             if isErasableBottomField(field, rhsClass) then erasedBottomTree(rhsClass)
             else transformFollowingDeep(ref(field))(using ctx.withOwner(sym))
           val getterDef = cpy.DefDef(tree)(rhs = getterRhs)
-          sym.copyAndKeepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot))
+          sym.keepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot))
           Thicket(fieldDef, getterDef)
       else if sym.isSetter then
         if (!sym.is(ParamAccessor)) { val Literal(Constant(())) = tree.rhs: @unchecked } // This is intended as an assertion
@@ -193,7 +193,7 @@ class Memoize extends MiniPhase with IdentityDenotTransformer { thisPhase =>
             then Literal(Constant(()))
             else Assign(ref(field), adaptToField(field, ref(tree.termParamss.head.head.symbol)))
           val setterDef = cpy.DefDef(tree)(rhs = transformFollowingDeep(initializer)(using ctx.withOwner(sym)))
-          sym.copyAndKeepAnnotationsCarrying(thisPhase, Set(defn.SetterMetaAnnot))
+          sym.keepAnnotationsCarrying(thisPhase, Set(defn.SetterMetaAnnot))
           setterDef
       else
         // Curiously, some accessors from Scala2 have ' ' suffixes.

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

@@ -157,14 +157,20 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           checkInferredWellFormed(tree.tpt)
           if sym.is(Method) then
             if sym.isSetter then
-              sym.copyAndKeepAnnotationsCarrying(thisPhase, Set(defn.SetterMetaAnnot))
+              sym.keepAnnotationsCarrying(thisPhase, Set(defn.SetterMetaAnnot))
+            if sym.isOneOf(GivenOrImplicit) then
+              val cls = sym.info.finalResultType.classSymbol
+              if cls.isOneOf(GivenOrImplicit) then
+                sym.updateAnnotationsAfter(thisPhase,
+                  atPhase(thisPhase)(cls.annotationsCarrying(Set(defn.CompanionMethodMetaAnnot)))
+                    ++ sym.annotations)
           else
             if sym.is(Param) then
-              sym.copyAndKeepAnnotationsCarrying(thisPhase, Set(defn.ParamMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
+              sym.keepAnnotationsCarrying(thisPhase, Set(defn.ParamMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
             else if sym.is(ParamAccessor) then
-              sym.copyAndKeepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot, defn.FieldMetaAnnot))
+              sym.keepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot, defn.FieldMetaAnnot))
             else
-              sym.copyAndKeepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot, defn.FieldMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
+              sym.keepAnnotationsCarrying(thisPhase, Set(defn.GetterMetaAnnot, defn.FieldMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
           if sym.isScala2Macro && !ctx.settings.XignoreScala2Macros.value then
             if !sym.owner.unforcedDecls.exists(p => !p.isScala2Macro && p.name == sym.name && p.signature == sym.signature)
                // Allow scala.reflect.materializeClassTag to be able to compile scala/reflect/package.scala
@@ -388,6 +394,8 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
             VarianceChecker.check(tree)
             annotateExperimental(sym)
             checkMacroAnnotation(sym)
+            if sym.isOneOf(GivenOrImplicit) then
+              sym.keepAnnotationsCarrying(thisPhase, Set(defn.CompanionClassMetaAnnot), orNoneOf = defn.MetaAnnots)
             tree.rhs match
               case impl: Template =>
                 for parent <- impl.parents do

tests/neg/i17002.scala

@@ -0,0 +1,20 @@
+import scala.annotation.compileTimeOnly
+
+sealed trait Test[T]
+
+object Test:
+  @compileTimeOnly("Error")
+  given test0[T]: Test[T] = ???
+
+  @compileTimeOnly("Error")
+  given test1[T]: Test[T]()
+
+  @compileTimeOnly("Error")
+  implicit class ic(x: Int):
+    def foo = 2
+
+  test0 // error
+
+  test1 // error
+
+  2.foo // error

tests/pos/i17002.scala

@@ -0,0 +1,10 @@
+import scala.annotation.meta.companionMethod
+
+@companionMethod
+class methOnly extends annotation.Annotation
+
+class Test
+object Test:
+
+  @methOnly
+  given test2[T]: Test with {}