metalang type bug

[Joonsoo]

floatLit: FloatLit = decimals '.' decimals? exponent? {FloatLiteral(integral=$0, fractional=$2, exponent:Exponent?=$3)}
  | decimals exponent {FloatLiteral($0, null, $1)}
  | '.' decimals exponent? {FloatLiteral("", $1, $2)}
  | "inf" {Inf()}
  | "nan" {NaN()}

FloatLiteral.fractional type is inferred to (string | string??). Expected behavior: (string | string?), which will be finally considered to be string?.