Generalize overriding checking from isDefined

Generalize overriding checking from isDefined
to all methods added by desugar to a case class.
None of these methods has an override so we
need to add one in case they do override another method
(previously we would flag this as an error).

Author: odersky

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

@@ -21,6 +21,13 @@ object desugar {
   /** Info of a variable in a pattern: The named tree and its type */
   private type VarInfo = (NameTree, Tree)
 
+  /** Names of methods that are added unconditionally to case classes */
+  def isDesugaredCaseClassMethodName(name: Name)(implicit ctx: Context) =
+    name == nme.isDefined ||
+    name == nme.copy ||
+    name == nme.productArity ||
+    name.isSelectorName
+
 // ----- DerivedTypeTrees -----------------------------------
 
   class SetterParamTree extends DerivedTypeTree {

src/dotty/tools/dotc/core/NameOps.scala

@@ -81,6 +81,7 @@ object NameOps {
     def isScala2LocalSuffix = name.endsWith(" ")
     def isModuleVarName(name: Name): Boolean =
       name.stripAnonNumberSuffix endsWith MODULE_VAR_SUFFIX
+    def isSelectorName = name.startsWith(" ") && name.tail.forall(_.isDigit)
 
     /** Is name a variable name? */
     def isVariableName: Boolean = name.length > 0 && {

src/dotty/tools/dotc/typer/RefChecks.scala

@@ -287,7 +287,7 @@ object RefChecks {
                  !member.isAnyOverride) {
         // (*) Exclusion for default getters, fixes SI-5178. We cannot assign the Override flag to
         // the default getter: one default getter might sometimes override, sometimes not. Example in comment on ticket.
-        if (member.name == nme.isDefined && member.is(Synthetic)) // isDefined methods are added automatially, can't have an override preset.
+        if (member.is(Synthetic) && desugar.isDesugaredCaseClassMethodName(member.name)) // such names are added automatically, can't have an override preset.
           member.setFlag(Override)
         else if (member.owner != clazz && other.owner != clazz && !(other.owner derivesFrom member.owner))
           emitOverrideError(