Compiler gives odd context in error that isn't relevant to the actual issue

[repnop]

Playground link: https://play.rust-lang.org/?gist=41596a413b9d567407a84423bd63174a&version=stable&mode=debug

Error:

error[E0308]: mismatched types
 --> src/main.rs:7:39
  |
1 | fn main() {
  |           - expected `()` because of default return type
...
7 |         thingy(if let Some(r) = opt { r } else { None });
  |                                       ^ expected enum `std::option::Option`, found mutable reference
  |
  = note: expected type `std::option::Option<&mut i32>`
             found type `&mut {integer}`

For some reason rustc appears to include the return type of main as if it was important here, even though the expression is passing the return value to the thingy function.

[estebank]

Duplicate of #46302.