disallow negative indexing

compiler/semmagic.nim

@@ -135,6 +135,13 @@ proc isStrangeArray(t: PType): bool =
   let t = t.skipTypes(abstractInst)
   result = t.kind == tyArray and t.firstOrd != 0
 
+proc isNegative(n: PNode): bool =
+  let n = n.skipConv
+  if n.kind in {nkCharLit..nkUInt64Lit}:
+    result = n.intVal < 0
+  elif n.kind in nkCallKinds and n.sons[0].kind == nkSym:
+    result = n.sons[0].sym.magic in {mUnaryMinusI, mUnaryMinusI64}
+
 proc magicsAfterOverloadResolution(c: PContext, n: PNode,
                                    flags: TExprFlags): PNode =
   case n[0].sym.magic
@@ -158,6 +165,12 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
   of mProcCall:
     result = n
     result.typ = n[1].typ
+  of mDotDot:
+    result = n
+    # we only need to warnings here about negative indexing:
+    if isNegative(n.sons[1]) or (n.len > 2 and isNegative(n.sons[2])):
+      message(n.info, warnDeprecated,
+        "use '^' instead of '-'; negative indexing")
   of mRoof:
     # error correction:
     result = n.sons[1]