Remove last redundant condition and compensate for it in individual tests

Author: odersky

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

@@ -360,31 +360,6 @@ object RefChecks {
 
       //Console.println(infoString(member) + " overrides " + infoString(other) + " in " + clazz);//DEBUG
 
-      // return if we already checked this combination elsewhere
-      if (member.owner != clazz) {
-        def deferredCheck = member.is(Deferred) || !other.is(Deferred)
-        def subOther(s: Symbol) = s derivesFrom other.owner
-        def subMember(s: Symbol) = s derivesFrom member.owner
-
-        if (subOther(member.owner) && deferredCheck)
-          //println(i"skip 1 ${member.showLocated}, ${other.showLocated}")
-          //Console.println(infoString(member) + " shadows1 " + infoString(other) " in " + clazz);//DEBUG
-          return
-        /*
-        val parentSymbols = clazz.info.parents.map(_.typeSymbol)
-        def matchIn(parent: Symbol): Boolean = considerMatching(member, other, parent.thisType)
-        if parentSymbols.exists(p =>
-          subOther(p) && subMember(p) && deferredCheck && matchIn(p))
-        then
-          println(i"skip 2 ${member.showLocated}, ${other.showLocated}")
-          //Console.println(infoString(member) + " shadows2 " + infoString(other) + " in " + clazz);//DEBUG
-          return
-        if parentSymbols.forall(p => subOther(p) == subMember(p) && matchIn(p)) then
-          println(i"skip 3 ${member.showLocated}, ${other.showLocated}")
-          //Console.println(infoString(member) + " shadows " + infoString(other) + " in " + clazz);//DEBUG
-          return*/
-      }
-
       /* Is the intersection between given two lists of overridden symbols empty? */
       def intersectionIsEmpty(syms1: Iterator[Symbol], syms2: Iterator[Symbol]) = {
         val set2 = syms2.toSet
@@ -419,7 +394,9 @@ object RefChecks {
         overrideError("cannot be used here - class definitions cannot be overridden")
       else if (!other.is(Deferred) && member.isClass)
         overrideError("cannot be used here - classes can only override abstract types")
-      else if (other.isEffectivelyFinal) // (1.2)
+      else if other.isEffectivelyFinal && !member.owner.derivesFrom(other.owner) then // (1.2)
+        // If member is in a subclass, we checked this before or the signatures are different
+        // This escape clause is needed to make MapView in stdlib213 compile
         overrideError(i"cannot override final member ${other.showLocated}")
       else if (member.is(ExtensionMethod) && !other.is(ExtensionMethod)) // (1.3)
         overrideError("is an extension method, cannot override a normal method")
@@ -440,15 +417,22 @@ object RefChecks {
           member.setFlag(Override)
         else if (member.isType && self.memberInfo(member) =:= self.memberInfo(other))
           () // OK, don't complain about type aliases which are equal
-        else if (member.owner != clazz && other.owner != clazz &&
-                 !(other.owner derivesFrom member.owner))
+        else if member.owner != clazz
+             && other.owner != clazz
+             && !other.owner.derivesFrom(member.owner)
+             && !member.owner.derivesFrom(other.owner)
+        then
           emitOverrideError(
             s"$clazz inherits conflicting members:\n  "
               + infoStringWithLocation(other) + "  and\n  " + infoStringWithLocation(member)
               + "\n(Note: this can be resolved by declaring an override in " + clazz + ".)")
         else if member.is(Exported) then
           overrideError("cannot override since it comes from an export")
-        else
+        else if !member.owner.derivesFrom(other.owner) then
+          // if `member.owner` derives from `other.owner` then the relationship was either
+          // checked before, or `member` and `other` have different signatures in `member.owner`,
+          // in which case we could not add an `override` anyway.
+          // This escape clause is needed to make MapView in stdlib213 compile
           overrideError("needs `override` modifier")
       else if (other.is(AbsOverride) && other.isIncompleteIn(clazz) && !member.is(AbsOverride))
         overrideError("needs `abstract override` modifiers")