Allow return type declaration for generated function

[cstjean]

Runtime dispatch to a generated function is always typed as ::Any, for good reason:

@generated function foo(obj)
    quote
        return true 
    end
end
glob = 2
bar() = foo(glob)
@inferred bar()  # error: return type Bool does not match inferred return type Any

If we could specify the return type @generated function foo(obj)::Bool, that would help fix common cases (eg. predicates). Currently, this syntax is valid, but it specifies the return type of the expansion, which is useless.

[JeffBezanson]

I see the problem, but currently return type declarations don't really work that way --- putting ::Bool on a function doesn't mean that we can or will infer Bool as the return type. It's syntax for a certain transformation of the function body. I agree it should be applied to the generated code, but we still won't be able to infer the return type without calling the generator.

[JeffBezanson]

#12783 is related; also about sorting out which stage things belong to.

[yuyichao]

Given that we add type assertion to after the convert we should be able to infer?

[JeffBezanson]

Yes, but if the argument type to the generated function isn't known at compile time, we can't call the generator, so there's no function body to look at.

[yuyichao]

Ok. I was assuming the representation/lowering of return type declaration will need changing for generated function since merely an automatic convertion doesn't seem to be too useful for generated function when the user can easily achieve this when generating the code.

[StefanKarpinski]

Given that the other type annotations in the function signature are used for dispatch, which means they're considered to mean what they mean at application time, it seems sane to me that the return type annotation would mean that as well. In which case it would presumably mean that each return point is automatically rewritten with the appropriate convert-assert, no?