import std/[typetraits, macros]

template vecBorrow() {.pragma.}

proc isMarked(T: typedesc): bool =
  hasCustomPragma(T, vecBorrow)

type
  Vec3 = array[3, float32]
  Color {.vecBorrow.} = distinct Vec3
  Point3 {.vecBorrow.} = distinct Vec3
  VecOps = concept type V
    distinctBase(V) is Vec3
    V.isMarked

proc `[]`[T: VecOps](c: var T, ind: int): var float32 = distinctBase(c)[ind]
proc `[]=`[T: VecOps](c: var T, ind: int, val: float32) = distinctBase(c)[ind] =
val
proc `[]`[T: VecOps](c: T, ind: int): float32 = distinctBase(c)[ind]
proc `$`[T: VecOps](c: T): string = $distinctBase(c)

var a = Color Vec3([1f, 2, 3])
a[0] = 300
echo a[0]
echo a
# For a maddening example