TAIT: `item does not constrain` after diverging expression

[narpfel]

I tried this code:

#![feature(type_alias_impl_trait)]

pub type Tait = impl Iterator<Item = u64>;

#[define_opaque(Tait)]
pub fn f() -> Tait {
    loop {}
    IntoIterator::into_iter([])
}

I expected to see this happen: This code should compile.

Instead, this happened: An error is generated.

warning: unreachable expression
 --> <source>:8:5
  |
7 |     loop {}
  |     ------- any code following this expression is unreachable
8 |     IntoIterator::into_iter([])
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable expression
  |
  = note: `#[warn(unreachable_code)]` on by default

error: item does not constrain `Tait::{opaque#0}`
 --> <source>:6:8
  |
6 | pub fn f() -> Tait {
  |        ^
  |
  = note: consider removing `#[define_opaque]` or adding an empty `#[define_opaque()]`
note: this opaque type is supposed to be constrained
 --> <source>:3:17
  |
3 | pub type Tait = impl Iterator<Item = u64>;
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error; 1 warning emitted

Compiler returned: 1

This happens when the function’s body contains an an unconditional diverging expression (such as loop {} or panic!()). If the diverging expression is wrapped in if true { ... }, the code compiles fine.

Godbolt link: https://godbolt.org/z/T65dMvG6o

Note that the same code compiles fine with -Z next-solver: https://godbolt.org/z/nsqT36bWE

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (6f6971078 2025-05-28)
binary: rustc
commit-hash: 6f69710780d579b180ab38da4c1384d630f7bd31
commit-date: 2025-05-28
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

Backtrace

<backtrace>