first attempt to fix 'a[i]' handling in generics

lib/system.nim

@@ -239,6 +239,14 @@ type
   seq*{.magic: "Seq".}[T]  ## Generic type to construct sequences.
   set*{.magic: "Set".}[T]  ## Generic type to construct bit sets.
 
+when defined(nimArrIdx):
+  # :array|openarray|string|seq|cstring|tuple
+  proc `[]`*[I: Ordinal;T](a: T; i: I): T {.
+    noSideEffect, magic: "ArrGet".}
+  proc `[]=`*[I: Ordinal;T,S](a: var T; i: I;
+    x: S) {.noSideEffect, magic: "ArrPut".}
+  proc `=`*[T](dest: var T; src: T) {.noSideEffect, magic: "Asgn".}
+
 type
   Slice*[T] = object ## builtin slice type
     a*, b*: T        ## the bounds