You don't even need to annotate things with types!

    > square(x) = x * x

    > @code_typed square(2.0)
    CodeInfo(
    1 1 ─ %1 = Base.mul_float(%%x, %%x)::Float64
    └──        return %1
    ) => Float64

    > @code_typed square(2)
    CodeInfo(
    1 1 ─ %1 = Base.mul_int(%%x, %%x)::Int64
    └──        return %1
    ) => Int64

Functions specialize automatically to the arguments you pass in.