Incorrect mismatch location reported by RegexParsers

implicit def literal(s: String): Parser[String] = new Parser[String] {
  def apply(in: Input) = {
    val source = in.source
    val offset = in.offset
    val start = handleWhiteSpace(source, offset)
    var i = 0
    var j = start
    while (i < s.length && j < source.length && s.charAt(i) == source.charAt(j)) {
      i += 1
      j += 1
    }
    if (i == s.length)
      Success(source.subSequence(start, j).toString, in.drop(j - offset), None)
    else  {
      val found = if (start == source.length()) "end of source" else "'"+source.charAt(start)+"'"
      Failure("'"+s+"' expected but "+found+" found", in.drop(start - offset))
    }
  }
}

Note the error reporting at the end where the index of the mismatch is reported as start. If the string is "apple", and the input "...apply...", the mismatch is at the character 'y', which is pointed to by the variable j, not at the character 'a', which is pointed to by start.

println(parse("apple", "apply"))

Failure('apple' expected but 'a' found,CharSequenceReader('a', ...))

A potential fix may be as follows:

val rest = in.drop(start - offset)
if (start == source.length()) Failure(s"'$s' expected but end of source found", rest)
else Failure(s"'$s' expected but '${source.charAt(j)}' found at index $j", rest)

Start with the implicit literal parser implementation shown in the issue and reproduce parse("apple", "apply"). Trace the mismatch through start and j, then verify that the reported failure identifies the mismatching character and location rather than the initial offset.