False positive warning in `unused_parens` lint with dyn Trait returned from a closure

[alexcrichton]

Code

I tried this code:

#![deny(unused_parens)]
#![allow(warnings)]
trait MyTrait {}

fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {}

fn main() {}

I expected compile with no warnings, but instead, the current nightly yields:

error: unnecessary parentheses around type
 --> src/main.rs:5:43
  |
5 | fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {}
  |                                           ^           ^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(unused_parens)]
  |         ^^^^^^^^^^^^^
help: remove these parentheses
  |
5 - fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {}
5 + fn foo(_: Box<dyn FnMut(&mut u32) -> &mut dyn MyTrait + Send + Sync>) {}
  |

error: aborting due to 1 previous error

Version with regression

Bisecting via cargo-bisect-rustc yields:

searched nightlies: from nightly-2025-07-02 to nightly-2025-07-08
regressed nightly: nightly-2025-07-04
searched commit range: 6677875...da58c05
regressed commit: 25face9

bisected with cargo-bisect-rustc v0.6.10

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc

where my guess is that #142237 is the cause (cc @benschulz as the author)

[alexcrichton]

Oh sorry, forgot to also mention, by removing the parens:

#![deny(unused_parens)]
#![allow(warnings)]
trait MyTrait {}

fn foo(_: Box<dyn FnMut(&mut u32) -> &mut dyn MyTrait + Send + Sync>) {}

fn main() {}

the compiler reports:

error: ambiguous `+` in a type
 --> src/main.rs:5:43
  |
5 | fn foo(_: Box<dyn FnMut(&mut u32) -> &mut dyn MyTrait + Send + Sync>) {}
  |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: try adding parentheses
  |
5 | fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait + Send + Sync)>) {}
  |                                           +                         +

error: aborting due to 1 previous error