Just curious. It looks like that something as Base.:+(a,b)= a*b has no effect.
Does Julia not allow redefining methods in Base? If so, what would be rationale behind it?
Do you really want to get 5+6 as 30?
Just guessing here, but you are probably just testing the effect incorrectly, e.g. Int + Int has a more special method that will be called even after your definition.
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No not really. 
If such thing is allowed, it would be possible to define such as Base.:+(a::CustomType, b::CustomType) as you wish.
Oh I see. Base.:+(a::CustomType, b::CustomType) does work indeed. Int + Int seems to be handled specially.
It’s not special, it’s multiple dispatch! One of Julia’s not-so-secret sauces Methods · The Julia Language
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You can do Base.:+(a::Int, b::Int) = a * b but Julia won’t survive long enough for you to enjoy it, since it totally breaks the internals.
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@kevbonham, @GunnarFarneback, A-ha, yes, indeed. Thanks!