Fix ignoring __index override by derived classes

operators.pluto

@@ -6,7 +6,7 @@ local Pluto_operator_new <const> = function(mt, ...)
   end
   local t = {}
   setmetatable(t, mt)
-  if not mt.__index or mt.__parent then
+  if not rawget(mt, "__index") then
     mt.__index = mt
   end
   if mt.__construct then

src/lparser.cpp

@@ -5788,16 +5788,15 @@ static void builtinoperators (LexState *ls) {
       ls->tokens.emplace_back(Token(TK_NAME, luaX_newliteral(ls, "mt")));
       ls->tokens.emplace_back(Token(')'));
 
-      //   if not mt.__index or mt.__parent then
+      //   if not rawget(mt, "__index") then
       ls->tokens.emplace_back(Token(TK_IF));
       ls->tokens.emplace_back(Token(TK_NOT));
+      ls->tokens.emplace_back(Token(TK_NAME, luaX_newliteral(ls, "rawget")));
+      ls->tokens.emplace_back(Token('('));
       ls->tokens.emplace_back(Token(TK_NAME, luaX_newliteral(ls, "mt")));
-      ls->tokens.emplace_back(Token('.'));
-      ls->tokens.emplace_back(Token(TK_NAME, luaX_newliteral(ls, "__index")));
-      ls->tokens.emplace_back(Token(TK_OR));
-      ls->tokens.emplace_back(Token(TK_NAME, luaX_newliteral(ls, "mt")));
-      ls->tokens.emplace_back(Token('.'));
-      ls->tokens.emplace_back(Token(TK_NAME, luaX_newliteral(ls, "__parent")));
+      ls->tokens.emplace_back(Token(','));
+      ls->tokens.emplace_back(Token(TK_STRING, luaX_newliteral(ls, "__index")));
+      ls->tokens.emplace_back(Token(')'));
       ls->tokens.emplace_back(Token(TK_THEN));
 
       //     mt.__index = mt

testes/pluto/basic.pluto

@@ -2853,7 +2853,7 @@ do
     local p = new Player("John")
     assert(p.name == "John")
 end
--- Class overriding '__index'
+-- Overriding '__index'
 do
     $define extraOffset = 10
 
@@ -2880,6 +2880,32 @@ do
     c.num = 42
     assert(c.num == 42)
 end
+do
+    local class Base
+        base_member = 1
+    end
+
+    local Derived
+    class Derived extends Base
+        derived_member = 2
+
+        function __index(key)
+            if key == "magic" then
+                return 3
+            end
+            return Derived[key]
+        end
+    end
+
+    local inst = new Derived()
+    assert(inst.base_member == 1)
+    assert(inst.derived_member == 2)
+    assert(inst.magic == 3)
+    inst.derived_member += 10
+    inst.user_defined = 4
+    assert(inst.derived_member == 12)
+    assert(inst.user_defined == 4)
+end
 -- Metamethod inheritance
 do
     local class Base