warning: the feature `type_alias_impl_trait` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/issue-52843-closure-constrain.rs:5:32
   |
LL | #![cfg_attr(full_tait, feature(type_alias_impl_trait))]
   |                                ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(incomplete_features)]` on by default
   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information

error: concrete type differs from previous defining opaque type use
  --> $DIR/issue-52843-closure-constrain.rs:13:16
   |
LL |     let null = || -> Opaque { 0 };
   |                ^^^^^^^^^^^^^^^^^^ expected `String`, got `i32`
   |
note: previous use here
  --> $DIR/issue-52843-closure-constrain.rs:12:5
   |
LL |     fn _unused() -> Opaque { String::new() }
   |     ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error; 1 warning emitted