_       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.6.0-dev.2153 (2017-01-18 05:52 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit b8971ea4f* (0 days old master)
|__/                   |  x86_64-pc-linux-gnu

julia> immutable TT{X,Y} end

julia> f{X,Y}(::Type{TT{X,Y}},x::X,y::Y) = 1
f (generic function with 1 method)

julia> f{X}(::Type{TT{X,X}},x::X) = 100
ERROR: StackOverflowError:

julia> f(TT{Int,Int},1,2)
1

julia> f(TT{Int,Int},1) # it still works fine!
100

julia> methods(f)
# 2 methods for generic function "f":
f{X}(::Type{TT{X,X}}, x::X) in Main at REPL[3]:1
f{X,Y}(::Type{TT{X,Y}}, x::X, y::Y) in Main at REPL[2]:1