Function declared_fn_value_type 

pub fn declared_fn_value_type(fn_id: u16, state: &State<'_>) -> DataType

The type a declared function has where a value holds it.

A function whose parameters and return are annotated says its own signature, so a call through the value is checked against what the declaration names. One the compiler infers says only which function it holds, and the first call through the value settles the types its body is compiled at. The function is a candidate of the type either way, which is what compiles the body a call through the value jumps to.