make var string return var char w/ BackwardsIndex (#15461)

* make var string return var char w/ BackwardsIndex

fixes #14497

* work around VM bug
* properly workaround again

lib/pure/pathnorm.nim

@@ -69,7 +69,7 @@ proc addNormalizePath*(x: string; result: var string; state: var int;
   while hasNext(it, x):
     let b = next(it, x)
     if (state shr 1 == 0) and isSlash(x, b):
-      if result.len == 0 or result[^1] notin {DirSep, AltSep}:
+      if result.len == 0 or result[result.len - 1] notin {DirSep, AltSep}:
         result.add dirSep
       state = state or 1
     elif isDotDot(x, b):
@@ -87,13 +87,13 @@ proc addNormalizePath*(x: string; result: var string; state: var int;
           setLen(result, d-1)
           dec state, 2
       else:
-        if result.len > 0 and result[^1] notin {DirSep, AltSep}:
+        if result.len > 0 and result[result.len - 1] notin {DirSep, AltSep}:
           result.add dirSep
         result.add substr(x, b[0], b[1])
     elif isDot(x, b):
       discard "discard the dot"
     elif b[1] >= b[0]:
-      if result.len > 0 and result[^1] notin {DirSep, AltSep}:
+      if result.len > 0 and result[result.len - 1] notin {DirSep, AltSep}:
         result.add dirSep
       result.add substr(x, b[0], b[1])
       inc state, 2

lib/system.nim

@@ -2579,6 +2579,7 @@ proc `[]`*[T](s: var openArray[T]; i: BackwardsIndex): var T {.inline.} =
   system.`[]`(s, s.len - int(i))
 proc `[]`*[Idx, T](a: var array[Idx, T]; i: BackwardsIndex): var T {.inline.} =
   a[Idx(a.len - int(i) + int low(a))]
+proc `[]`*(s: var string; i: BackwardsIndex): var char {.inline.} = s[s.len - int(i)]
 
 proc `[]=`*[T](s: var openArray[T]; i: BackwardsIndex; x: T) {.inline.} =
   system.`[]=`(s, s.len - int(i), x)

tests/stdlib/tstring.nim

@@ -91,3 +91,10 @@ proc tester[T](x: T) =
 
 tester(1)
 
+# #14497 
+func reverse*(a: string): string =
+  result = a
+  for i in 0 ..< a.len div 2:
+    swap(result[i], result[^(i + 1)])
+
+doAssert reverse("hello") == "olleh"