Julia: Dispatch on Pair

[jw3126]

   _       _ _(_)_     |  A fresh approach to technical computing
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  | | |_| | | | (_| |  |  Version 0.6.0-dev.975 (2016-10-24 19:00 UTC)
 _/ |\__'_|_|_|\__'_|  |  scan/0929f08* (fork: 5 commits, 16 days)
|__/                   |  x86_64-linux-gnu

julia> f{K}(::Pair{K}) = 1
f (generic function with 1 method)

julia> f{K, V}(::Pair{K, V}) = 2
f (generic function with 2 methods)

julia> f("a"=>123)
1

[JeffBezanson]

I can already confirm this will be fixed by #8974, after which the two signatures are type-equal and the second definition overwrites the first (with usual warning).

[JeffBezanson]

Dup of #18892