diff --git a/tests/ui/delegation/not-supported.rs b/tests/ui/delegation/not-supported.rs
index 5f78de97638..25d7a4cb895 100644
--- a/tests/ui/delegation/not-supported.rs
+++ b/tests/ui/delegation/not-supported.rs
@@ -70,12 +70,16 @@ mod opaque {
 
         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
         pub fn opaque_ret() -> impl Trait { unimplemented!() }
+        //~^ warn: this function depends on never type fallback being `()`
+        //~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     }
     reuse to_reuse::opaque_arg;
     //~^ ERROR delegation with early bound generics is not supported yet
 
     trait ToReuse {
         fn opaque_ret() -> impl Trait { unimplemented!() }
+        //~^ warn: this function depends on never type fallback being `()`
+        //~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     }
 
     // FIXME: Inherited `impl Trait`s create query cycles when used inside trait impls.
diff --git a/tests/ui/delegation/not-supported.stderr b/tests/ui/delegation/not-supported.stderr
index e2cb04f977b..339a8418b33 100644
--- a/tests/ui/delegation/not-supported.stderr
+++ b/tests/ui/delegation/not-supported.stderr
@@ -107,7 +107,7 @@ LL |         reuse Trait::foo2 { &self.0 }
    |                      ^^^^
 
 error: delegation with early bound generics is not supported yet
-  --> $DIR/not-supported.rs:74:21
+  --> $DIR/not-supported.rs:76:21
    |
 LL |         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
    |         --------------------------------------- callee defined here
@@ -115,46 +115,67 @@ LL |         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
 LL |     reuse to_reuse::opaque_arg;
    |                     ^^^^^^^^^^
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>::{synthetic#0}`
-  --> $DIR/not-supported.rs:83:25
+warning: this function depends on never type fallback being `()`
+  --> $DIR/not-supported.rs:80:9
+   |
+LL |         fn opaque_ret() -> impl Trait { unimplemented!() }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:86:5: 86:24>::{synthetic#0}`
+  --> $DIR/not-supported.rs:87:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:83:25
+  --> $DIR/not-supported.rs:87:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>` is well-formed
-  --> $DIR/not-supported.rs:82:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:86:5: 86:24>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:86:5: 86:24>` is well-formed
+  --> $DIR/not-supported.rs:86:5
    |
 LL |     impl ToReuse for u8 {
    |     ^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>::{synthetic#0}`
-  --> $DIR/not-supported.rs:86:24
+warning: this function depends on never type fallback being `()`
+  --> $DIR/not-supported.rs:72:9
+   |
+LL |         pub fn opaque_ret() -> impl Trait { unimplemented!() }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:89:5: 89:25>::{synthetic#0}`
+  --> $DIR/not-supported.rs:90:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:86:24
+  --> $DIR/not-supported.rs:90:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>` is well-formed
-  --> $DIR/not-supported.rs:85:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:89:5: 89:25>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:89:5: 89:25>` is well-formed
+  --> $DIR/not-supported.rs:89:5
    |
 LL |     impl ToReuse for u16 {
    |     ^^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
 error: recursive delegation is not supported yet
-  --> $DIR/not-supported.rs:99:22
+  --> $DIR/not-supported.rs:103:22
    |
 LL |         pub reuse to_reuse2::foo;
    |                              --- callee defined here
@@ -162,7 +183,7 @@ LL |         pub reuse to_reuse2::foo;
 LL |     reuse to_reuse1::foo;
    |                      ^^^
 
-error: aborting due to 16 previous errors
+error: aborting due to 16 previous errors; 2 warnings emitted
 
 Some errors have detailed explanations: E0049, E0195, E0391.
 For more information about an error, try `rustc --explain E0049`.