invalid ccall/cfunction signature mismatch

[vtjnash]

You're declaring @cfunction with type Ref{Minifier}, but calling it from ccall with type Ref{Minifier{IOBuffer}}. This is invalid: you must pass the same type in both places. One valid options would be to use Any in both places. Another valid option would be to use Ref{T} in both places (as is already being done in ccall) and allocating the specialized cfunction for the actual Context instance by defining as such:

function cb_map_start(ctx::T) where T <: Context
    return @cfunction on_map_start Cint (Ref{T},)
end