fixes #6692

lib/system.nim

@@ -3527,18 +3527,20 @@ proc `[]=`*[T, U, V](s: var seq[T], x: HSlice[U, V], b: openArray[T]) =
   else:
     spliceImpl(s, a, L, b)
 
-proc `[]`*[T](s: openArray[T]; i: BackwardsIndex): T {.inline.} = s[s.len - int(i)]
+proc `[]`*[T](s: openArray[T]; i: BackwardsIndex): T {.inline.} =
+  system.`[]`(s, s.len - int(i))
+
 proc `[]`*[Idx, T](a: array[Idx, T]; i: BackwardsIndex): T {.inline.} =
   a[Idx(a.len - int(i) + int low(a))]
 proc `[]`*(s: string; i: BackwardsIndex): char {.inline.} = s[s.len - int(i)]
 
 proc `[]`*[T](s: var openArray[T]; i: BackwardsIndex): var T {.inline.} =
-  s[s.len - int(i)]
+  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 `[]=`*[T](s: var openArray[T]; i: BackwardsIndex; x: T) {.inline.} =
-  s[s.len - int(i)] = x
+  system.`[]=`(s, s.len - int(i), x)
 proc `[]=`*[Idx, T](a: var array[Idx, T]; i: BackwardsIndex; x: T) {.inline.} =
   a[Idx(a.len - int(i) + int low(a))] = x
 proc `[]=`*(s: var string; i: BackwardsIndex; x: char) {.inline.} =

tests/array/troofregression.nim

@@ -0,0 +1,46 @@
+###############################
+#### part from Arraymancer
+
+type
+  MetadataArray* = object
+    data*: array[8, int]
+    len*: int
+
+# Commenting the converter removes the error "lib/system.nim(3536, 3) Error: for a 'var' type a variable needs to be passed"
+converter toMetadataArray*(se: varargs[int]): MetadataArray {.inline.} =
+  result.len = se.len
+  for i in 0..<se.len:
+    result.data[i] = se[i]
+
+
+when NimVersion >= "0.17.3":
+  type Index = int or BackwardsIndex
+  template `^^`(s, i: untyped): untyped =
+    when i is BackwardsIndex:
+      s.len - int(i)
+    else: i
+else:
+  type Index = int
+  template `^^`(s, i: untyped): untyped =
+    i
+
+## With Nim devel from the start of the week (~Oct30) I managed to trigger "lib/system.nim(3536, 4) Error: expression has no address"
+## but I can't anymore after updating Nim (Nov5)
+## Now commenting this plain compiles and removes the error "lib/system.nim(3536, 3) Error: for a 'var' type a variable needs to be passed"
+proc `[]`*(a: var MetadataArray, idx: Index): var int {.inline.} =
+  a.data[a ^^ idx]
+
+
+##############################
+### Completely unrelated lib that triggers the issue
+
+type
+  MySeq[T] = ref object
+    data: seq[T]
+
+proc test[T](sx: MySeq[T]) =
+  # Removing the backward index removes the error "lib/system.nim(3536, 3) Error: for a 'var' type a variable needs to be passed"
+  echo sx.data[^1] # error here
+
+let s = MySeq[int](data: @[1, 2, 3])
+s.test()