Question mark in `gen fn` doesn't work.

I tried this code (playground link):

#![feature(gen_blocks)]

gen fn fallible() -> Option<u32> {
    None?;
}

I expected to see this happen: The code compiles and fallible() produces an iterator that yields exactly one None. As far as I understand, this is prescribed by RFC 3513 and #117078 says the RFC has been implemented.

Instead, this happened: There's a compilation error:

error[E0277]: the `?` operator can only be used in a gen function that returns `Result` or `Option` (or another type that implements `FromResidual`)
 --> src/main.rs:4:9
  |
3 |   gen fn fallible() -> Option<u32> {
  |  __________________________________-
4 | |     None?;
  | |         ^ cannot use the `?` operator in a gen function that returns `()`
5 | | }
  | |_- this function should return `Result` or `Option` to accept `?`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Note that the function does "return" an Option. But as I understand the diagnostics here are expected to be unfinished so perhaps the exact wording of the error is not so important.

Meta

rustc --version --verbose:

rustc 1.98.0-nightly (f28ac764c 2026-06-23)
binary: rustc
commit-hash: f28ac764c36004fa6a6e098d15b4016a838c13c6
commit-date: 2026-06-23
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.7

or 1.98.0-nightly (2026-06-23 f28ac764c36004fa6a6e) on playground.

Start by reproducing the linked Playground example and read RFC 3513's error-handling section alongside issue #117078. Trace the compiler handling for ? in a gen fn; done means the example compiles, fallible() yields exactly one None, and the relevant diagnostic or regression test reflects the supported behavior.