Make main methods invisible

Add a new flag Invisible that gets set for compiler-generated main methods.
Invisible symbols are skipped when resolving members during typechecking.
They become visible after typechecking.

With that mechanism, we can accept main methods that are themselves called `main`.

Author: odersky

compiler/src/dotty/tools/dotc/ast/MainProxies.scala

@@ -39,13 +39,6 @@ object MainProxies {
     mainMethods(stats).flatMap(mainProxy)
   }
 
-  private def checkNoShadowing(mainFun: Symbol)(using Context) =
-    val cls = ctx.typer.findRef(mainFun.name.toTypeName, WildcardType, EmptyFlags, EmptyFlags, mainFun).typeSymbol
-    if cls.exists && cls.owner != ctx.owner then
-      report.warning(
-        i"""The class `${ctx.printer.fullNameString(mainFun)}` generated from `@main` will shadow the existing ${cls.showLocated}.
-           |The existing definition might no longer be found on recompile.""", mainFun)
-
   import untpd._
   def mainProxy(mainFun: Symbol)(using Context): List[TypeDef] = {
     val mainAnnotSpan = mainFun.getAnnotation(defn.MainAnnot).get.tree.span
@@ -93,7 +86,6 @@ object MainProxies {
         case _ =>
           report.error(s"@main can only annotate a method", pos)
       }
-      checkNoShadowing(mainFun)
       val errVar = Ident(nme.error)
       val handler = CaseDef(
         Typed(errVar, TypeTree(defn.CLP_ParseError.typeRef)),
@@ -106,7 +98,7 @@ object MainProxies {
         .withFlags(JavaStatic)
       val mainTempl = Template(emptyConstructor, Nil, Nil, EmptyValDef, mainMeth :: Nil)
       val mainCls = TypeDef(mainFun.name.toTypeName, mainTempl)
-        .withFlags(Final)
+        .withFlags(Final | Invisible)
       if (!ctx.reporter.hasErrors) result = mainCls.withSpan(mainAnnotSpan.toSynthetic) :: Nil
     }
     result

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

@@ -1061,11 +1061,12 @@ object Denotations {
     def filterDisjoint(denots: PreDenotation)(using Context): SingleDenotation =
       if (denots.exists && denots.matches(this)) NoDenotation else this
     def filterWithFlags(required: FlagSet, excluded: FlagSet)(using Context): SingleDenotation =
+      val realExcluded = if ctx.isAfterTyper then excluded else excluded | Invisible
       def symd: SymDenotation = this match
         case symd: SymDenotation => symd
         case _ => symbol.denot
       if !required.isEmpty && !symd.isAllOf(required)
-         || !excluded.isEmpty && symd.isOneOf(excluded) then NoDenotation
+         || symd.isOneOf(realExcluded) then NoDenotation
       else this
     def aggregate[T](f: SingleDenotation => T, g: (T, T) => T): T = f(this)
 

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

@@ -369,6 +369,9 @@ object Flags {
   /** An infix method or type */
   val (Infix @ _, _, _) = newFlags(44, "infix")
 
+  /** Symbol cannot be found as a member during typer */
+  val (Invisible @ _, _, _) = newFlags(45, "<invisible>")
+
   // ------------ Flags following this one are not pickled ----------------------------------
 
   /** Symbol is not a member of its owner */
@@ -458,7 +461,7 @@ object Flags {
     Scala2SpecialFlags, MutableOrOpen, Opaque, Touched, JavaStatic,
     OuterOrCovariant, LabelOrContravariant, CaseAccessor,
     Extension, NonMember, Implicit, Given, Permanent, Synthetic,
-    SuperParamAliasOrScala2x, Inline, Macro, ConstructorProxy)
+    SuperParamAliasOrScala2x, Inline, Macro, ConstructorProxy, Invisible)
 
   /** Flags that are not (re)set when completing the denotation, or, if symbol is
    *  a top-level class or object, when completing the denotation once the class
@@ -512,7 +515,7 @@ object Flags {
   val RetainedModuleValAndClassFlags: FlagSet =
     AccessFlags | Package | Case |
     Synthetic | JavaDefined | JavaStatic | Artifact |
-    Lifted | MixedIn | Specialized | ConstructorProxy
+    Lifted | MixedIn | Specialized | ConstructorProxy | Invisible
 
   /** Flags that can apply to a module val */
   val RetainedModuleValFlags: FlagSet = RetainedModuleValAndClassFlags |

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

@@ -572,8 +572,9 @@ object SymDenotations {
         isAbsent(canForce)
       case _ =>
         // Otherwise, no completion is necessary, see the preconditions of `markAbsent()`.
-        (myInfo `eq` NoType) ||
-        is(ModuleVal, butNot = Package) && moduleClass.isAbsent(canForce)
+        (myInfo `eq` NoType)
+        || is(Invisible) && !ctx.isAfterTyper
+        || is(ModuleVal, butNot = Package) && moduleClass.isAbsent(canForce)
     }
 
     /** Is this symbol the root class or its companion object? */
@@ -2209,8 +2210,8 @@ object SymDenotations {
         ensureCompleted()
       myCompanion
 
-    override def registeredCompanion_=(c: Symbol) = 
-      myCompanion = c 
+    override def registeredCompanion_=(c: Symbol) =
+      myCompanion = c
 
     private var myNestingLevel = -1
 

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -724,6 +724,7 @@ class TreePickler(pickler: TastyPickler) {
     if (flags.is(Artifact)) writeModTag(ARTIFACT)
     if flags.is(Transparent) then writeModTag(TRANSPARENT)
     if flags.is(Infix) then writeModTag(INFIX)
+    if flags.is(Invisible) then writeModTag(INVISIBLE)
     if (isTerm) {
       if (flags.is(Implicit)) writeModTag(IMPLICIT)
       if (flags.is(Given)) writeModTag(GIVEN)

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -670,6 +670,7 @@ class TreeUnpickler(reader: TastyReader,
           case PARAMalias => addFlag(SuperParamAlias)
           case EXPORTED => addFlag(Exported)
           case OPEN => addFlag(Open)
+          case INVISIBLE => addFlag(Invisible)
           case TRANSPARENT => addFlag(Transparent)
           case INFIX => addFlag(Infix)
           case PRIVATEqualified =>

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -109,7 +109,8 @@ class CompilationTests {
       compileFile("tests/pos/i0239.scala", defaultOptions),
       compileFile("tests/pos/anonClassSubtyping.scala", defaultOptions),
       compileFile("tests/pos/extmethods.scala", defaultOptions),
-      compileFile("tests/pos/companions.scala", defaultOptions)
+      compileFile("tests/pos/companions.scala", defaultOptions),
+      compileFile("tests/pos/main.scala", defaultOptions)
     ).times(2).checkCompile()
   }
 

tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -211,7 +211,8 @@ Standard-Section: "ASTs" TopLevelStat*
                   PARAMsetter                                                      -- The setter part `x_=` of a var parameter `x` which itself is pickled as a PARAM
                   PARAMalias                                                       -- Parameter is alias of a superclass parameter
                   EXPORTED                                                         -- An export forwarder
-                  OPEN                                                             -- an open class
+                  OPEN
+                  INVISIBLE                                                            -- an open class
                   Annotation
 
   Variance      = STABLE                                                           -- invariant
@@ -284,7 +285,7 @@ object TastyFormat {
    * compatibility, but remains backwards compatible, with all
    * preceeding `MinorVersion`.
    */
-  final val MinorVersion: Int = 0
+  final val MinorVersion: Int = 1
 
   /**Natural Number. The `ExperimentalVersion` allows for
    * experimentation with changes to TASTy without committing
@@ -474,6 +475,7 @@ object TastyFormat {
   final val INFIX = 43
   final val EMPTYCLAUSE = 44
   final val SPLITCLAUSE = 45
+  final val INVISIBLE = 46
 
   // Cat. 2:    tag Nat
 
@@ -636,6 +638,7 @@ object TastyFormat {
        | PARAMalias
        | EXPORTED
        | OPEN
+       | INVISIBLE
        | ANNOTATION
        | PRIVATEqualified
        | PROTECTEDqualified => true
@@ -698,6 +701,7 @@ object TastyFormat {
     case PARAMsetter => "PARAMsetter"
     case EXPORTED => "EXPORTED"
     case OPEN => "OPEN"
+    case INVISIBLE => "INVISIBLE"
     case PARAMalias => "PARAMalias"
     case EMPTYCLAUSE => "EMPTYCLAUSE"
     case SPLITCLAUSE => "SPLITCLAUSE"

tests/neg-custom-args/fatal-warnings/i10137.scala

@@ -0,0 +1,4 @@
+package foo:
+  @main def main(): Unit = println("Hello, World!")
+
+@main def List(): Unit = println("List")