close #7097 add testcase (#16682)

tests/converter/t7097.nim

@@ -0,0 +1,38 @@
+type
+  Byte* = uint8
+  Bytes* = seq[Byte]
+  
+  BytesRange* = object
+    bytes: Bytes
+    ibegin, iend: int
+
+proc initBytesRange*(s: var Bytes, ibegin = 0, iend = -1): BytesRange =
+  let e = if iend < 0: s.len + iend + 1
+          else: iend
+  assert ibegin > 0 and e <= s.len
+  
+  shallow(s)
+  result.bytes = s
+  result.ibegin = ibegin
+  result.iend = e
+
+template `[]=`*(r: var BytesRange, i: int, v: Byte) =
+  r.bytes[r.ibegin + i] = v
+
+converter fromSeq*(s: Bytes): BytesRange =
+  var seqCopy = s
+  return initBytesRange(seqCopy)
+
+type
+  Reader* = object
+    data: BytesRange
+    position: int
+
+proc readerFromHex*(input: string): Reader =
+  let totalBytes = input.len div 2
+  var backingStore = newSeq[Byte](totalBytes)
+  result.data = initBytesRange(backingStore)
+
+  for i in 0 ..< totalBytes:
+    var nextByte = 0
+    result.data[i] = Byte(nextByte) # <-------- instantiated from here