Function definition order matters with generated functions

[omus]

Any function that is called within a generated function needs to be defined before the generated function definition otherwise we encounter a MethodError from the world being too new:

julia> f1() = 1
f1 (generic function with 1 method)

julia> @generated function g1()
           v = f1()
           quote
               return $v
           end
       end
g1 (generic function with 1 method)

julia> g1()
1

vs.

julia> @generated function g2()
           v = f2()
           quote
               return $v
           end
       end
g2 (generic function with 1 method)

julia> f2() = 1
f2 (generic function with 1 method)

julia> g2()
ERROR: MethodError: no method matching f2()
The applicable method may be too new: running in world age 21396, while current world is 21397.
Closest candidates are:
  f2() at REPL[5]:1 (method too new to be called from this world context.)
Stacktrace:
 [1] g2(...) at ./REPL[4]:2

Introduced with #17057

[omus]

cc: @vtjnash

[yuyichao]

I believe this is documented.

[yuyichao]

Basically dup of #19942. Closing this and any discussion about it can continue on that issue.

[omus]

For completeness. Documented in Metaprogramming:

Some operations that should not be attempted include:
....
Calling any function that is defined after the body of the generated function.