Rationalize() error

Specific Domains Numerics
1 
               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: https://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.7.0-DEV.3404 (2018-01-14 21:52 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit d569a2923c* (8 days old master)
|__/                   |  x86_64-w64-mingw32

julia> pi
π = 3.1415926535897...

julia> big(pi)
3.141592653589793238462643383279502884197169399375105820974944592307816406286198

julia> rationalize(Int128,pi)
60728338969805745700507212595448411044//19330430665609526556707216376512714945

julia> rationalize(Int128,pi,0.000001)
ERROR: MethodError: no method matching rationalize(::Type{Int128}, ::Irrational{:π}, ::Float64)
Closest candidates are:
  rationalize(::Type{T<:Integer}, ::AbstractFloat, ::Real) where T<:Integer at rational.jl:127
  rationalize(::Type{T}, ::AbstractIrrational; tol) where T at irrationals.jl:81
  rationalize(::Type{T<:Integer}, ::AbstractFloat; tol) where T<:Integer at rational.jl:185

julia>
2

The tolerance is a keyword argument:

julia> rationalize(Int128, pi, tol=0.000001)
355//113

That’s what arguments after a semicolon in the method signature mean:

help?> rationalize
search: rationalize Rational Irrational AbstractIrrational

  rationalize([T<:Integer=Int,] x; tol::Real=eps(x))

  Approximate floating point number x as a Rational number with components
  of the given integer type. The result will differ from x by no more than
  tol.

  julia> rationalize(5.6)
  28//5

  julia> a = rationalize(BigInt, 10.3)
  103//10

  julia> typeof(numerator(a))
  BigInt
3

Sorry to remember this:
https://github.com/JuliaLang/julia/pull/16143