C11's _Generic-like expressions in julia with ccall and an application

_generic.jl

@generated function _Generic(t1::Type{T1}, args...) where T1
    for i in 1:2:length(args)
        T2 = args[i].parameters[1]
        Ex = args[i+1].parameters[1].parameters[1]
        
        isa(T2,Type) || throw(ArgumentError("$T2 is not a type"))
        
        if Ex == :_
            T1 <: T2 && return T1
        else
            Ex = isa(Ex, Symbol) ? QuoteNode(Expr(:quote, Ex)) : Ex
            T1 <: T2 && return Ex
        end
    end
    throw(ArgumentError("No matching type found"))
end

macro generic(t, pairs...)
    args = [map(pair->[pair.args[2], :($Val{$(pair.args[3])})], pairs)...;]
    esc(:($_Generic($t, $(args...))))
end

generic_tryparse.jl

function tryparse(::Type{T}, as::AbstractString) where T<:Union{Float32, Float64}
    s = String(as)
    s[1] == '_' && return Nullable{T}()
    s = replace(s, "_", "")

    ccall(@generic(T,
              Float32 => :jl_try_substrtof,
              Float64 => :jl_try_substrtod),
          Nullable{T}, (Ptr{UInt8}, Csize_t, Csize_t), s, 0, sizeof(s))
end