Allow static class methods as main methods

Author: odersky

compiler/src/dotty/tools/dotc/config/JavaPlatform.scala

@@ -22,10 +22,13 @@ class JavaPlatform extends Platform {
 
   // The given symbol is a method with the right name and signature to be a runnable java program.
   def isMainMethod(sym: SymDenotation)(implicit ctx: Context): Boolean =
-    (sym.name == nme.main) && (sym.info match {
-      case MethodTpe(_, defn.ArrayOf(el) :: Nil, restpe) => el =:= defn.StringType && (restpe isRef defn.UnitClass)
-      case _ => false
-    })
+    sym.name == nme.main &&
+    (sym.owner.is(Module) || sym.owner.isClass && !sym.owner.is(Trait) && sym.is(JavaStatic)) && {
+      sym.info match {
+        case MethodTpe(_, defn.ArrayOf(el) :: Nil, restpe) => el =:= defn.StringType && (restpe isRef defn.UnitClass)
+        case _ => false
+      }
+    }
 
   /** Update classpath with a substituted subentry */
   def updateClassPath(subst: Map[ClassPath, ClassPath]): Unit = currentClassPath.get match {

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -230,7 +230,7 @@ private class ExtractAPICollector(implicit val ctx: Context) extends ThunkHolder
 
     allNonLocalClassesInSrc += cl
 
-    if (sym.isStatic && defType == DefinitionType.Module && ctx.platform.hasMainMethod(sym)) {
+    if (sym.isStatic && ctx.platform.hasMainMethod(sym)) {
       _mainClasses += name
     }