11 files changed, 95 insertions, 23 deletions
diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr b/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
new file mode 100644
index 00000000000..fbaa874792a
--- /dev/null
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
@@ -0,0 +1,19 @@
+error[E0277]: the trait bound `(): std::error::Error` is not satisfied
+  --> $DIR/coerce-issue-49593-box-never.rs:17:53
+   |
+LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
+   |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+   |
+   = note: required for the cast to the object type `dyn std::error::Error`
+
+error[E0277]: the trait bound `(): std::error::Error` is not satisfied
+  --> $DIR/coerce-issue-49593-box-never.rs:22:49
+   |
+LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
+   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+   |
+   = note: required for the cast to the object type `(dyn std::error::Error + 'static)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never.rs b/src/test/ui/coercion/coerce-issue-49593-box-never.rs
index 0824ce8cd58..7a4324bd5ad 100644
--- a/src/test/ui/coercion/coerce-issue-49593-box-never.rs
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never.rs
@@ -1,5 +1,9 @@
-// check-pass
-#![feature(never_type, never_type_fallback)]
+// revisions: nofallback fallback
+//[fallback] check-pass
+//[nofallback] check-fail
+
+#![feature(never_type)]
+#![cfg_attr(fallback, feature(never_type_fallback))]
 #![allow(unreachable_code)]
 
 use std::error::Error;
@@ -11,10 +15,12 @@ fn raw_ptr_box<T>(t: T) -> *mut T {
 
 fn foo(x: !) -> Box<dyn Error> {
     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
+    //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn foo_raw_ptr(x: !) -> *mut dyn Error {
     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
+    //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn no_coercion(d: *mut dyn Error) -> *mut dyn Error {
diff --git a/src/test/ui/never_type/defaulted-never-note.stderr b/src/test/ui/never_type/defaulted-never-note.fallback.stderr
index 109a81a5ca0..588d644c77b 100644
--- a/src/test/ui/never_type/defaulted-never-note.stderr
+++ b/src/test/ui/never_type/defaulted-never-note.fallback.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: ImplementedForUnitButNotNever` is not satisfied
-  --> $DIR/defaulted-never-note.rs:26:5
+  --> $DIR/defaulted-never-note.rs:30:5
    |
 LL |     foo(_x);
    |     ^^^ the trait `ImplementedForUnitButNotNever` is not implemented for `!`
@@ -8,7 +8,7 @@ LL |     foo(_x);
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information).
    = help: did you intend to use the type `()` here instead?
 note: required by a bound in `foo`
-  --> $DIR/defaulted-never-note.rs:21:11
+  --> $DIR/defaulted-never-note.rs:25:11
    |
 LL | fn foo<T: ImplementedForUnitButNotNever>(_t: T) {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `foo`
diff --git a/src/test/ui/never_type/defaulted-never-note.rs b/src/test/ui/never_type/defaulted-never-note.rs
index 70333c5f324..54f551759cb 100644
--- a/src/test/ui/never_type/defaulted-never-note.rs
+++ b/src/test/ui/never_type/defaulted-never-note.rs
@@ -1,6 +1,10 @@
+// revisions: nofallback fallback
+//[nofallback] run-pass
+//[fallback] check-fail
+
 // We need to opt into the `never_type_fallback` feature
 // to trigger the requirement that this is testing.
-#![feature(never_type, never_type_fallback)]
+#![cfg_attr(fallback, feature(never_type, never_type_fallback))]
 
 #![allow(unused)]
 
@@ -19,16 +23,16 @@ trait ImplementedForUnitButNotNever {}
 impl ImplementedForUnitButNotNever for () {}
 
 fn foo<T: ImplementedForUnitButNotNever>(_t: T) {}
-//~^ NOTE required by this bound in `foo`
-//~| NOTE required by a bound in `foo`
+//[fallback]~^ NOTE required by this bound in `foo`
+//[fallback]~| NOTE required by a bound in `foo`
 fn smeg() {
     let _x = return;
     foo(_x);
-    //~^ ERROR the trait bound
-    //~| NOTE the trait `ImplementedForUnitButNotNever` is not implemented
-    //~| NOTE this trait is implemented for `()`
-    //~| NOTE this error might have been caused
-    //~| HELP did you intend
+    //[fallback]~^ ERROR the trait bound
+    //[fallback]~| NOTE the trait `ImplementedForUnitButNotNever` is not implemented
+    //[fallback]~| NOTE this trait is implemented for `()`
+    //[fallback]~| NOTE this error might have been caused
+    //[fallback]~| HELP did you intend
 }
 
 fn main() {
diff --git a/src/test/ui/never_type/diverging-fallback-control-flow.rs b/src/test/ui/never_type/diverging-fallback-control-flow.rs
index f323f40ba31..45a3362fa6d 100644
--- a/src/test/ui/never_type/diverging-fallback-control-flow.rs
+++ b/src/test/ui/never_type/diverging-fallback-control-flow.rs
@@ -1,3 +1,4 @@
+// revisions: nofallback fallback
 // run-pass
 
 #![allow(dead_code)]
@@ -8,7 +9,7 @@
 // to fallback based on control-flow. In all of these cases,
 // the type variable winds up being the target of both a `!` coercion
 // and a coercion from a non-`!` variable, and hence falls back to `()`.
-#![feature(never_type, never_type_fallback)]
+#![cfg_attr(fallback, feature(never_type, never_type_fallback))]
 
 trait UnitDefault {
     fn default() -> Self;
diff --git a/src/test/ui/never_type/diverging-fallback-no-leak.stderr b/src/test/ui/never_type/diverging-fallback-no-leak.fallback.stderr
index 27615fe4e77..3a5b602f111 100644
--- a/src/test/ui/never_type/diverging-fallback-no-leak.stderr
+++ b/src/test/ui/never_type/diverging-fallback-no-leak.fallback.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: Test` is not satisfied
-  --> $DIR/diverging-fallback-no-leak.rs:14:5
+  --> $DIR/diverging-fallback-no-leak.rs:17:5
    |
 LL |     unconstrained_arg(return);
    |     ^^^^^^^^^^^^^^^^^ the trait `Test` is not implemented for `!`
@@ -8,7 +8,7 @@ LL |     unconstrained_arg(return);
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information).
    = help: did you intend to use the type `()` here instead?
 note: required by a bound in `unconstrained_arg`
-  --> $DIR/diverging-fallback-no-leak.rs:9:25
+  --> $DIR/diverging-fallback-no-leak.rs:12:25
    |
 LL | fn unconstrained_arg<T: Test>(_: T) {}
    |                         ^^^^ required by this bound in `unconstrained_arg`
diff --git a/src/test/ui/never_type/diverging-fallback-no-leak.rs b/src/test/ui/never_type/diverging-fallback-no-leak.rs
index a3a15f0ed88..03478e19ddc 100644
--- a/src/test/ui/never_type/diverging-fallback-no-leak.rs
+++ b/src/test/ui/never_type/diverging-fallback-no-leak.rs
@@ -1,4 +1,7 @@
-#![feature(never_type_fallback)]
+// revisions: nofallback fallback
+//[nofallback] check-pass
+
+#![cfg_attr(fallback, feature(never_type, never_type_fallback))]
 
 fn make_unit() {}
 
@@ -10,6 +13,7 @@ fn unconstrained_arg<T: Test>(_: T) {}
 
 fn main() {
     // Here the type variable falls back to `!`,
-    // and hence we get a type error:
-    unconstrained_arg(return); //~ ERROR trait bound `!: Test` is not satisfied
+    // and hence we get a type error.
+    unconstrained_arg(return);
+    //[fallback]~^ ERROR trait bound `!: Test` is not satisfied
 }
diff --git a/src/test/ui/never_type/diverging-fallback-unconstrained-return.rs b/src/test/ui/never_type/diverging-fallback-unconstrained-return.rs
index 9a6c965cefb..7ea97126f89 100644
--- a/src/test/ui/never_type/diverging-fallback-unconstrained-return.rs
+++ b/src/test/ui/never_type/diverging-fallback-unconstrained-return.rs
@@ -6,7 +6,10 @@
 //
 // check-pass
 
-#![feature(never_type_fallback)]
+// revisions: nofallback fallback
+
+#![cfg_attr(fallback, feature(never_type, never_type_fallback))]
+
 
 fn make_unit() {}
 
diff --git a/src/test/ui/never_type/fallback-closure-wrap.fallback.stderr b/src/test/ui/never_type/fallback-closure-wrap.fallback.stderr
new file mode 100644
index 00000000000..78d1a3caf4a
--- /dev/null
+++ b/src/test/ui/never_type/fallback-closure-wrap.fallback.stderr
@@ -0,0 +1,17 @@
+error[E0271]: type mismatch resolving `<[closure@$DIR/fallback-closure-wrap.rs:18:40: 21:6] as FnOnce<()>>::Output == ()`
+  --> $DIR/fallback-closure-wrap.rs:18:31
+   |
+LL |       let error = Closure::wrap(Box::new(move || {
+   |  _______________________________^
+LL | |
+LL | |         panic!("Can't connect to server.");
+LL | |     }) as Box<dyn FnMut()>);
+   | |______^ expected `()`, found `!`
+   |
+   = note: expected unit type `()`
+                   found type `!`
+   = note: required for the cast to the object type `dyn FnMut()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.
diff --git a/src/test/ui/never_type/never-value-fallback-issue-66757.nofallback.stderr b/src/test/ui/never_type/never-value-fallback-issue-66757.nofallback.stderr
new file mode 100644
index 00000000000..f374266626b
--- /dev/null
+++ b/src/test/ui/never_type/never-value-fallback-issue-66757.nofallback.stderr
@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `E: From<()>` is not satisfied
+  --> $DIR/never-value-fallback-issue-66757.rs:27:5
+   |
+LL |     <E as From<_>>::from(never);
+   |     ^^^^^^^^^^^^^^^^^^^^ the trait `From<()>` is not implemented for `E`
+   |
+   = help: the following implementations were found:
+             <E as From<!>>
+note: required by `from`
+  --> $SRC_DIR/core/src/convert/mod.rs:LL:COL
+   |
+LL |     fn from(_: T) -> Self;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/never_type/never-value-fallback-issue-66757.rs b/src/test/ui/never_type/never-value-fallback-issue-66757.rs
index f2e9e087307..6dc7e6ad2d9 100644
--- a/src/test/ui/never_type/never-value-fallback-issue-66757.rs
+++ b/src/test/ui/never_type/never-value-fallback-issue-66757.rs
@@ -4,12 +4,13 @@
 // never) and an uninferred variable (here the argument to `From`) it
 // doesn't fallback to `()` but rather `!`.
 //
-// run-pass
+// revisions: nofallback fallback
+//[fallback] run-pass
+//[nofallback] check-fail
 
 #![feature(never_type)]
 
-// FIXME(#67225) -- this should be true even without the fallback gate.
-#![feature(never_type_fallback)]
+#![cfg_attr(fallback, feature(never_type_fallback))]
 
 struct E;
 
@@ -23,7 +24,7 @@ impl From<!> for E {
 #[allow(dead_code)]
 fn foo(never: !) {
     <E as From<!>>::from(never);  // Ok
-    <E as From<_>>::from(never);  // Inference fails here
+    <E as From<_>>::from(never);  //[nofallback]~ ERROR trait bound `E: From<()>` is not satisfied
 }
 
 fn main() { }