Unclear when `Fn.return_type` should be null

[MasonRemaley]

Sometimes, Fn.return_type is set to null. It's unclear what the the rule is here, there's currently a TODO in the docs:

TODO change the language spec to make this not optional.

I figured it was worth filing an issue to track either resolving this TODO, or replacing it with an explanation of when the value will be null and when it won't.

Feel free to close if this is a dup--I searched a bit but wasn't able to find anything, but there were a lot of false positives so I could've missed it.

---

Here's an example of a case where it ends up being null right now:

pub fn temp(a: anytype) @TypeOf(a) {
    std.debug.assert(@typeInfo(@TypeOf(temp)).@"fn".return_type.? == f32);
    return 0.0;
}

test temp {
    _ = temp(@as(f32, 0.0));
}

The real use case that lead to me wanting to do this is here. I was trying to avoid duplicating the logic that decides the return type, but pulling it out into a function is a bit messy since @TypeOf (for good reason) takes values not types so I'd end up having to do @TypeOf(@as(undefined, T)) etc.

[linusg]

Related: #2935

[rohlem]

I was trying to avoid duplicating the logic that decides the return type

Maybe a bit off-topic, but you might be interested in the TypedGet hack I came up with
(it might be overkill here, for now, but in my code it's certainly been useful):

(brief summary/example)

Basically you can write a function that takes the (relevant) argument types
and returns a namespace with declarations const Result: type and fn get(...) Result {...} (the actual implementation),
then make a proxy function that forwards to that implementation:

pub fn lerp_get(comptime Start: type, comptime End: type) type {
  return opaque {
    pub const Result = ...; //decide the result type
    pub fn get(start: Start, end: End, t: anytype) Result {
      ... // actual implementation
    }
  };
}

pub fn lerp(start: anytype, end: anytype, t: anytype) lerp_get(@TypeOf(start), @TypeOf(end)).Result {
  return lerp_get(@TypeOf(start), @TypeOf(end)).get(start, end, t);
}