I think you may have some misconceptions. Julia is not compiled ahead of time. A function does not spring into existence until the definition of the function is executed. Same applies to a struct definition.
In a statically typed language you might write a function that takes an integer like this `foo(x::Int)` but in Julia it is completely possible to replace the `Int` with an expression that returns a type object. Meaning you actually run code at runtime to figure out what the defined function should actually operate on.
So there is nothing known at compile time, because everything happens at runtime. However when a function is called for the first time, then the JIT compiler will compile it and store the result. But it will store different compiled versions of the same function depending on the type of the argument it was called with. If you call the same function later with the same type of argument, it will reuse a previously compiled version.
In a statically typed language you might write a function that takes an integer like this `foo(x::Int)` but in Julia it is completely possible to replace the `Int` with an expression that returns a type object. Meaning you actually run code at runtime to figure out what the defined function should actually operate on.
So there is nothing known at compile time, because everything happens at runtime. However when a function is called for the first time, then the JIT compiler will compile it and store the result. But it will store different compiled versions of the same function depending on the type of the argument it was called with. If you call the same function later with the same type of argument, it will reuse a previously compiled version.
If this is all very unclear. You could look at an article I wrote contrasting static typing with how typing works in Julia: https://medium.com/@Jernfrost/types-in-c-c-and-julia-ce0fcbe...