Disallow self types in objects

Fixes #11648

Author: odersky

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

@@ -890,12 +890,9 @@ object desugar {
       val modul = ValDef(moduleName, clsRef, New(clsRef, Nil))
         .withMods(mods.toTermFlags & RetainedModuleValFlags | ModuleValCreationFlags)
         .withSpan(mdef.span.startPos)
-      val ValDef(selfName, selfTpt, _) = impl.self
-      val selfMods = impl.self.mods
-      if (!selfTpt.isEmpty) report.error(ObjectMayNotHaveSelfType(mdef), impl.self.srcPos)
-      val clsSelf = ValDef(selfName, SingletonTypeTree(Ident(moduleName)), impl.self.rhs)
-        .withMods(selfMods)
-        .withSpan(impl.self.span.orElse(impl.span.startPos))
+      if !impl.self.isEmpty then report.error(ObjectMayNotHaveSelfType(mdef), impl.self.srcPos)
+      val clsSelf = ValDef(nme.WILDCARD, SingletonTypeTree(Ident(moduleName)), EmptyTree)
+        .withSpan(impl.span.startPos)
       val clsTmpl = cpy.Template(impl)(self = clsSelf, body = impl.body)
       val cls = TypeDef(clsName, clsTmpl)
         .withMods(mods.toTypeFlags & RetainedModuleClassFlags | ModuleClassCreationFlags)

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -452,21 +452,9 @@ import transform.SymUtils._
   }
 
   class ObjectMayNotHaveSelfType(mdef: untpd.ModuleDef)(using Context)
-  extends SyntaxMsg(ObjectMayNotHaveSelfTypeID) {
-    def msg = em"""${hl("object")}s must not have a self ${hl("type")}"""
-
-    def explain = {
-      val untpd.ModuleDef(name, tmpl) = mdef
-      val ValDef(_, selfTpt, _) = tmpl.self
-      em"""|${hl("object")}s must not have a self ${hl("type")}:
-           |
-           |Consider these alternative solutions:
-           |  - Create a trait or a class instead of an object
-           |  - Let the object extend a trait containing the self type:
-           |
-           |    object $name extends ${selfTpt.show}"""
-    }
-  }
+  extends SyntaxMsg(ObjectMayNotHaveSelfTypeID):
+    def msg = "objects must not have a self type"
+    def explain = ""
 
   class RepeatedModifier(modifier: String)(implicit ctx:Context)
   extends SyntaxMsg(RepeatedModifierID) {

tests/neg/i11648.scala

@@ -0,0 +1,5 @@
+class A {
+  object B { b => // error
+
+  }
+}