Rollup merge of #124080 - oli-obk:define_opaque_types10, r=compiler-errors

Some unstable changes to where opaque types get defined

None of these can be reached from stable afaict.

r? ``@compiler-errors``

cc https://github.com/rust-lang/rust/issues/116652

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2539,7 +2539,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         let InferOk { obligations, .. } = self
             .infcx
             .at(&cause, obligation.param_env)
-            .eq(DefineOpaqueTypes::No, placeholder_obligation_trait_ref, impl_trait_ref)
+            .eq(DefineOpaqueTypes::Yes, placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| {
                 debug!("match_impl: failed eq_trait_refs due to `{}`", e.to_string(self.tcx()))
             })?;
@@ -2594,7 +2594,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                         self.infcx
                             .at(&obligation.cause, obligation.param_env)
                             .eq(
-                                DefineOpaqueTypes::No,
+                                DefineOpaqueTypes::Yes,
                                 upcast_principal.map_bound(|trait_ref| {
                                     ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
                                 }),
@@ -2631,7 +2631,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                     nested.extend(
                         self.infcx
                             .at(&obligation.cause, obligation.param_env)
-                            .eq(DefineOpaqueTypes::No, source_projection, target_projection)
+                            .eq(DefineOpaqueTypes::Yes, source_projection, target_projection)
                             .map_err(|_| SelectionError::Unimplemented)?
                             .into_obligations(),
                     );

tests/ui/impl-trait/equality.rs

@@ -22,7 +22,7 @@ fn sum_to(n: u32) -> impl Foo {
         0
     } else {
         n + sum_to(n - 1)
-        //~^ ERROR cannot add `impl Foo` to `u32`
+        //~^ ERROR cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
     }
 }
 
@@ -32,12 +32,15 @@ trait Leak: Sized {
 }
 impl<T> Leak for T {
     default type T = ();
-    default fn leak(self) -> Self::T { panic!() }
+    default fn leak(self) -> Self::T {
+        panic!()
+    }
 }
 impl Leak for i32 {
     type T = i32;
-    fn leak(self) -> i32 { self }
+    fn leak(self) -> i32 {
+        self
+    }
 }
 
-fn main() {
-}
+fn main() {}

tests/ui/impl-trait/equality.stderr

@@ -22,20 +22,13 @@ help: change the type of the numeric literal from `u32` to `i32`
 LL |     0_i32
    |       ~~~
 
-error[E0277]: cannot add `impl Foo` to `u32`
+error[E0284]: type annotations needed: cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
   --> $DIR/equality.rs:24:11
    |
 LL |         n + sum_to(n - 1)
-   |           ^ no implementation for `u32 + impl Foo`
-   |
-   = help: the trait `Add<impl Foo>` is not implemented for `u32`
-   = help: the following other types implement trait `Add<Rhs>`:
-             <&'a u32 as Add<u32>>
-             <&u32 as Add<&u32>>
-             <u32 as Add<&u32>>
-             <u32 as Add>
+   |           ^ cannot satisfy `<u32 as Add<impl Foo>>::Output == i32`
 
 error: aborting due to 2 previous errors; 1 warning emitted
 
-Some errors have detailed explanations: E0277, E0308.
-For more information about an error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0284, E0308.
+For more information about an error, try `rustc --explain E0284`.

tests/ui/impl-trait/nested_impl_trait.stderr

@@ -46,19 +46,23 @@ error[E0277]: the trait bound `impl Into<u32>: Into<impl Debug>` is not satisfie
   --> $DIR/nested_impl_trait.rs:6:46
    |
 LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Into<u32>`, which is required by `impl Into<u32>: Into<impl Debug>`
+   |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `impl Into<u32>`
    |
-   = help: the trait `Into<U>` is implemented for `T`
-   = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
+help: consider further restricting this bound
+   |
+LL | fn bad_in_ret_position(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
+   |                                          +++++++++++++++++
 
 error[E0277]: the trait bound `impl Into<u32>: Into<impl Debug>` is not satisfied
   --> $DIR/nested_impl_trait.rs:19:34
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Into<u32>`, which is required by `impl Into<u32>: Into<impl Debug>`
+   |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `impl Into<u32>`
    |
-   = help: the trait `Into<U>` is implemented for `T`
-   = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
+help: consider further restricting this bound
+   |
+LL |     fn bad(x: impl Into<u32> + std::fmt::Debug) -> impl Into<impl Debug> { x }
+   |                              +++++++++++++++++
 
 error: aborting due to 7 previous errors
 

tests/ui/impl-trait/recursive-type-alias-impl-trait-declaration.rs

@@ -11,7 +11,7 @@ impl PartialEq<(Bar, i32)> for Bar {
 }
 
 fn foo() -> Foo {
-    //~^ ERROR can't compare `Bar` with `(Foo, i32)`
+    //~^ ERROR overflow evaluating the requirement `Bar: PartialEq<(Foo, i32)>`
     Bar
 }
 

tests/ui/impl-trait/recursive-type-alias-impl-trait-declaration.stderr

@@ -1,15 +1,9 @@
-error[E0277]: can't compare `Bar` with `(Foo, i32)`
+error[E0275]: overflow evaluating the requirement `Bar: PartialEq<(Foo, i32)>`
   --> $DIR/recursive-type-alias-impl-trait-declaration.rs:13:13
    |
 LL | fn foo() -> Foo {
-   |             ^^^ no implementation for `Bar == (Foo, i32)`
-LL |
-LL |     Bar
-   |     --- return type was inferred to be `Bar` here
-   |
-   = help: the trait `PartialEq<(Foo, i32)>` is not implemented for `Bar`
-   = help: the trait `PartialEq<(Bar, i32)>` is implemented for `Bar`
+   |             ^^^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0275`.

tests/ui/impl-trait/unsize_adt.rs

@@ -0,0 +1,14 @@
+//! Test that we do not allow unsizing `Foo<[Opaque; N]>` to `Foo<[Concrete]>`.
+
+struct Foo<T: ?Sized>(T);
+
+fn hello() -> Foo<[impl Sized; 2]> {
+    if false {
+        let x = hello();
+        let _: &Foo<[i32]> = &x;
+        //~^ ERROR: mismatched types
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/unsize_adt.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/unsize_adt.rs:8:30
+   |
+LL | fn hello() -> Foo<[impl Sized; 2]> {
+   |                    ---------- the found opaque type
+...
+LL |         let _: &Foo<[i32]> = &x;
+   |                -----------   ^^ expected `&Foo<[i32]>`, found `&Foo<[impl Sized; 2]>`
+   |                |
+   |                expected due to this
+   |
+   = note: expected reference `&Foo<[i32]>`
+              found reference `&Foo<[impl Sized; 2]>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/impl-trait/unsize_slice.rs

@@ -0,0 +1,12 @@
+//! Test that we do not allow unsizing `[Opaque; N]` to `[Concrete]`.
+
+fn hello() -> [impl Sized; 2] {
+    if false {
+        let x = hello();
+        let _: &[i32] = &x;
+        //~^ ERROR: mismatched types
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/unsize_slice.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/unsize_slice.rs:6:25
+   |
+LL | fn hello() -> [impl Sized; 2] {
+   |                ---------- the found opaque type
+...
+LL |         let _: &[i32] = &x;
+   |                ------   ^^ expected `&[i32]`, found `&[impl Sized; 2]`
+   |                |
+   |                expected due to this
+   |
+   = note: expected reference `&[i32]`
+              found reference `&[impl Sized; 2]`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/impl-trait/unsize_tuple.rs

@@ -0,0 +1,14 @@
+//! Test that we do not allow unsizing `([Opaque; N],)` to `([Concrete],)`.
+
+#![feature(unsized_tuple_coercion)]
+
+fn hello() -> ([impl Sized; 2],) {
+    if false {
+        let x = hello();
+        let _: &([i32],) = &x;
+        //~^ ERROR: mismatched types
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/unsize_tuple.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/unsize_tuple.rs:8:28
+   |
+LL | fn hello() -> ([impl Sized; 2],) {
+   |                 ---------- the found opaque type
+...
+LL |         let _: &([i32],) = &x;
+   |                ---------   ^^ expected `&([i32],)`, found `&([impl Sized; 2],)`
+   |                |
+   |                expected due to this
+   |
+   = note: expected reference `&([i32],)`
+              found reference `&([impl Sized; 2],)`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/trait-upcasting/illegal-upcast-to-impl-opaque.rs

@@ -0,0 +1,28 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@[next] failure-status: 101
+//@[next] known-bug: unknown
+//@[next] normalize-stderr-test "note: .*\n\n" -> ""
+//@[next] normalize-stderr-test "thread 'rustc' panicked.*\n.*\n" -> ""
+//@[next] normalize-stderr-test "(error: internal compiler error: [^:]+):\d+:\d+: " -> "$1:LL:CC: "
+//@[next] normalize-stderr-test "delayed at .*" -> ""
+//@[next] rustc-env:RUST_BACKTRACE=0
+//@ check-pass
+
+#![feature(trait_upcasting)]
+
+trait Super {
+    type Assoc;
+}
+
+trait Sub: Super {}
+
+impl<T: ?Sized> Super for T {
+    type Assoc = i32;
+}
+
+fn illegal(x: &dyn Sub<Assoc = i32>) -> &dyn Super<Assoc = impl Sized> {
+    x
+}
+
+fn main() {}

tests/ui/traits/trait-upcasting/type-checking-test-4.rs

@@ -11,6 +11,10 @@ fn test_correct(x: &dyn Foo<'static>) {
     let _ = x as &dyn Bar<'static, 'static>;
 }
 
+fn test_correct2<'a>(x: &dyn Foo<'a>) {
+    let _ = x as &dyn Bar<'_, '_>;
+}
+
 fn test_wrong1<'a>(x: &dyn Foo<'static>, y: &'a u32) {
     let _ = x as &dyn Bar<'static, 'a>; // Error
                                         //~^ ERROR lifetime may not live long enough

tests/ui/traits/trait-upcasting/type-checking-test-4.stderr

@@ -1,5 +1,5 @@
 error: lifetime may not live long enough
-  --> $DIR/type-checking-test-4.rs:15:13
+  --> $DIR/type-checking-test-4.rs:19:13
    |
 LL | fn test_wrong1<'a>(x: &dyn Foo<'static>, y: &'a u32) {
    |                -- lifetime `'a` defined here
@@ -7,7 +7,7 @@ LL |     let _ = x as &dyn Bar<'static, 'a>; // Error
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
 
 error: lifetime may not live long enough
-  --> $DIR/type-checking-test-4.rs:20:13
+  --> $DIR/type-checking-test-4.rs:24:13
    |
 LL | fn test_wrong2<'a>(x: &dyn Foo<'static>, y: &'a u32) {
    |                -- lifetime `'a` defined here
@@ -15,7 +15,7 @@ LL |     let _ = x as &dyn Bar<'a, 'static>; // Error
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
 
 error: lifetime may not live long enough
-  --> $DIR/type-checking-test-4.rs:26:5
+  --> $DIR/type-checking-test-4.rs:30:5
    |
 LL | fn test_wrong3<'a>(x: &dyn Foo<'a>) -> Option<&'static u32> {
    |                -- lifetime `'a` defined here
@@ -24,7 +24,7 @@ LL |     y.get_b() // ERROR
    |     ^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
 
 error: lifetime may not live long enough
-  --> $DIR/type-checking-test-4.rs:31:5
+  --> $DIR/type-checking-test-4.rs:35:5
    |
 LL | fn test_wrong4<'a>(x: &dyn Foo<'a>) -> Option<&'static u32> {
    |                -- lifetime `'a` defined here
@@ -32,7 +32,7 @@ LL |     <_ as Bar>::get_b(x) // ERROR
    |     ^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
 
 error: lifetime may not live long enough
-  --> $DIR/type-checking-test-4.rs:36:5
+  --> $DIR/type-checking-test-4.rs:40:5
    |
 LL | fn test_wrong5<'a>(x: &dyn Foo<'a>) -> Option<&'static u32> {
    |                -- lifetime `'a` defined here
@@ -40,7 +40,7 @@ LL |     <_ as Bar<'_, '_>>::get_b(x) // ERROR
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
 
 error: lifetime may not live long enough
-  --> $DIR/type-checking-test-4.rs:44:5
+  --> $DIR/type-checking-test-4.rs:48:5
    |
 LL | fn test_wrong6<'a>(x: &dyn Foo<'a>) -> Option<&'static u32> {
    |                -- lifetime `'a` defined here

tests/ui/traits/trait-upcasting/type-checking-test-opaques.rs

@@ -0,0 +1,22 @@
+#![feature(trait_upcasting, type_alias_impl_trait)]
+
+//@ check-pass
+
+type Tait = impl Sized;
+
+trait Foo<'a>: Bar<'a, 'a, Tait> {}
+trait Bar<'a, 'b, T> {}
+
+fn test_correct(x: &dyn Foo<'static>) {
+    let _ = x as &dyn Bar<'static, 'static, Tait>;
+}
+
+fn test_correct2<'a>(x: &dyn Foo<'a>) {
+    let _ = x as &dyn Bar<'_, '_, Tait>;
+}
+
+fn test_correct3<'a>(x: &dyn Foo<'a>, _: Tait) {
+    let _ = x as &dyn Bar<'_, '_, ()>;
+}
+
+fn main() {}

tests/ui/traits/trait-upcasting/upcast-defining-opaque.current.stderr

@@ -1,17 +0,0 @@
-error[E0308]: mismatched types
-  --> $DIR/upcast-defining-opaque.rs:21:5
-   |
-LL | type Foo = impl Sized;
-   |            ---------- the found opaque type
-LL |
-LL | fn upcast(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-   |                                        ----------------------- expected `&dyn Super<Assoc = i32>` because of return type
-LL |     x
-   |     ^ expected trait `Super`, found trait `Sub`
-   |
-   = note: expected reference `&dyn Super<Assoc = i32>`
-              found reference `&dyn Sub<Assoc = Foo>`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/trait-upcasting/upcast-defining-opaque.rs

@@ -1,7 +1,7 @@
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
 //@ ignore-compare-mode-next-solver (explicit revisions)
-//@[next] check-pass
+//@check-pass
 
 #![feature(trait_upcasting, type_alias_impl_trait)]
 
@@ -18,7 +18,7 @@ impl<T: ?Sized> Super for T {
 type Foo = impl Sized;
 
 fn upcast(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-    x //[current]~ mismatched types
+    x
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/constrain_in_projection.rs

@@ -0,0 +1,27 @@
+//! Check that projections will constrain opaque types while looking for
+//! matching impls.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@check-pass
+
+#![feature(type_alias_impl_trait)]
+
+struct Foo;
+
+type Bar = impl Sized;
+
+trait Trait<T> {
+    type Assoc: Default;
+}
+
+impl Trait<()> for Foo {
+    type Assoc = u32;
+}
+
+fn bop(_: Bar) {
+    let x = <Foo as Trait<Bar>>::Assoc::default();
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/constrain_in_projection2.current.stderr

@@ -0,0 +1,19 @@
+error[E0283]: type annotations needed: cannot satisfy `Foo: Trait<Bar>`
+  --> $DIR/constrain_in_projection2.rs:27:14
+   |
+LL |     let x = <Foo as Trait<Bar>>::Assoc::default();
+   |              ^^^ help: use the fully qualified path to an implementation: `<Type as Trait>::Assoc`
+   |
+note: multiple `impl`s satisfying `Foo: Trait<Bar>` found
+  --> $DIR/constrain_in_projection2.rs:18:1
+   |
+LL | impl Trait<()> for Foo {
+   | ^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | impl Trait<u32> for Foo {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: associated types cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.

tests/ui/type-alias-impl-trait/constrain_in_projection2.next.stderr

@@ -0,0 +1,19 @@
+error[E0283]: type annotations needed: cannot satisfy `Foo: Trait<Bar>`
+  --> $DIR/constrain_in_projection2.rs:27:14
+   |
+LL |     let x = <Foo as Trait<Bar>>::Assoc::default();
+   |              ^^^ help: use the fully qualified path to an implementation: `<Type as Trait>::Assoc`
+   |
+note: multiple `impl`s satisfying `Foo: Trait<Bar>` found
+  --> $DIR/constrain_in_projection2.rs:18:1
+   |
+LL | impl Trait<()> for Foo {
+   | ^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | impl Trait<u32> for Foo {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   = note: associated types cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.

tests/ui/type-alias-impl-trait/constrain_in_projection2.rs

@@ -0,0 +1,31 @@
+//! Check that projections will constrain opaque types while looking for
+//! matching impls and error if ambiguous.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+#![feature(type_alias_impl_trait)]
+
+struct Foo;
+
+type Bar = impl Sized;
+
+trait Trait<T> {
+    type Assoc: Default;
+}
+
+impl Trait<()> for Foo {
+    type Assoc = u32;
+}
+
+impl Trait<u32> for Foo {
+    type Assoc = u32;
+}
+
+fn bop(_: Bar) {
+    let x = <Foo as Trait<Bar>>::Assoc::default();
+    //~^ ERROR: cannot satisfy `Foo: Trait<Bar>`
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/issue-84660-unsoundness.current.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `Trait<Bar, _>`
-  --> $DIR/issue-84660-unsoundness.rs:23:1
+  --> $DIR/issue-84660-unsoundness.rs:28:1
    |
 LL | impl<In, Out> Trait<Bar, In> for Out {
    | ------------------------------------ first implementation here

tests/ui/type-alias-impl-trait/issue-84660-unsoundness.next.stderr

@@ -0,0 +1,23 @@
+error[E0284]: type annotations needed: cannot satisfy `<Out as Trait<Bar, In>>::Out == ()`
+  --> $DIR/issue-84660-unsoundness.rs:22:37
+   |
+LL |       fn convert(_i: In) -> Self::Out {
+   |  _____________________________________^
+LL | |
+LL | |         unreachable!();
+LL | |     }
+   | |_____^ cannot satisfy `<Out as Trait<Bar, In>>::Out == ()`
+
+error[E0119]: conflicting implementations of trait `Trait<Bar, _>`
+  --> $DIR/issue-84660-unsoundness.rs:28:1
+   |
+LL | impl<In, Out> Trait<Bar, In> for Out {
+   | ------------------------------------ first implementation here
+...
+LL | impl<In, Out> Trait<(), In> for Out {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0284.
+For more information about an error, try `rustc --explain E0119`.

tests/ui/type-alias-impl-trait/issue-84660-unsoundness.rs

@@ -1,6 +1,10 @@
 // Another example from issue #84660, this time weaponized as a safe transmute: an opaque type in an
 // impl header being accepted was used to create unsoundness.
 
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
 #![feature(type_alias_impl_trait)]
 
 trait Foo {}
@@ -16,11 +20,13 @@ trait Trait<T, In> {
 impl<In, Out> Trait<Bar, In> for Out {
     type Out = Out;
     fn convert(_i: In) -> Self::Out {
+        //[next]~^ ERROR: type annotations needed
         unreachable!();
     }
 }
 
-impl<In, Out> Trait<(), In> for Out { //~ ERROR conflicting implementations of trait `Trait<Bar, _>`
+impl<In, Out> Trait<(), In> for Out {
+    //~^ ERROR conflicting implementations of trait `Trait<Bar, _>`
     type Out = In;
     fn convert(i: In) -> Self::Out {
         i

tests/ui/type-alias-impl-trait/nested-tait-inference.rs

@@ -1,18 +1,21 @@
 #![feature(type_alias_impl_trait)]
 #![allow(dead_code)]
 
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@check-pass
+
 use std::fmt::Debug;
 
 type FooX = impl Debug;
 
-trait Foo<A> { }
+trait Foo<A> {}
 
-impl Foo<()> for () { }
+impl Foo<()> for () {}
 
 fn foo() -> impl Foo<FooX> {
-    //~^ ERROR: the trait bound `(): Foo<FooX>` is not satisfied
-    // FIXME(type-alias-impl-trait): We could probably make this work.
     ()
 }
 
-fn main() { }
+fn main() {}

tests/ui/type-alias-impl-trait/nested-tait-inference2.current.stderr

@@ -0,0 +1,17 @@
+error[E0283]: type annotations needed: cannot satisfy `(): Foo<FooX>`
+  --> $DIR/nested-tait-inference2.rs:17:13
+   |
+LL | fn foo() -> impl Foo<FooX> {
+   |             ^^^^^^^^^^^^^^
+   |
+note: multiple `impl`s satisfying `(): Foo<FooX>` found
+  --> $DIR/nested-tait-inference2.rs:14:1
+   |
+LL | impl Foo<()> for () {}
+   | ^^^^^^^^^^^^^^^^^^^
+LL | impl Foo<u32> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.

tests/ui/type-alias-impl-trait/nested-tait-inference2.next.stderr

@@ -0,0 +1,9 @@
+error[E0284]: type annotations needed: cannot satisfy `impl Foo<FooX> == ()`
+  --> $DIR/nested-tait-inference2.rs:19:5
+   |
+LL |     ()
+   |     ^^ cannot satisfy `impl Foo<FooX> == ()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0284`.

tests/ui/type-alias-impl-trait/nested-tait-inference2.rs

@@ -1,6 +1,10 @@
 #![feature(type_alias_impl_trait)]
 #![allow(dead_code)]
 
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
 use std::fmt::Debug;
 
 type FooX = impl Debug;
@@ -11,8 +15,9 @@ impl Foo<()> for () {}
 impl Foo<u32> for () {}
 
 fn foo() -> impl Foo<FooX> {
-    //~^ ERROR: the trait bound `(): Foo<FooX>` is not satisfied
+    //[current]~^ ERROR: cannot satisfy `(): Foo<FooX>`
     ()
+    //[next]~^ ERROR: cannot satisfy `impl Foo<FooX> == ()`
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/nested-tait-inference2.stderr

@@ -1,16 +0,0 @@
-error[E0277]: the trait bound `(): Foo<FooX>` is not satisfied
-  --> $DIR/nested-tait-inference2.rs:13:13
-   |
-LL | fn foo() -> impl Foo<FooX> {
-   |             ^^^^^^^^^^^^^^ the trait `Foo<FooX>` is not implemented for `()`
-LL |
-LL |     ()
-   |     -- return type was inferred to be `()` here
-   |
-   = help: the following other types implement trait `Foo<A>`:
-             <() as Foo<()>>
-             <() as Foo<u32>>
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0277`.

tests/ui/type-alias-impl-trait/normalize-hidden-types.current.stderr

@@ -22,21 +22,17 @@ note: previous use here
 LL |     fn define_1() -> Opaque { dyn_hoops::<_>(0) }
    |                               ^^^^^^^^^^^^^^^^^
 
-error[E0308]: mismatched types
+error: concrete type differs from previous defining opaque type use
   --> $DIR/normalize-hidden-types.rs:43:25
    |
-LL |     type Opaque = impl Sized;
-   |                   ---------- the expected opaque type
-...
 LL |         let _: Opaque = dyn_hoops::<u8>(0);
-   |                ------   ^^^^^^^^^^^^^^^^^^ expected opaque type, found `*const dyn FnOnce(())`
-   |                |
-   |                expected due to this
+   |                         ^^^^^^^^^^^^^^^^^^ expected `*const (dyn FnOnce(()) + 'static)`, got `*const dyn for<'a> FnOnce(<u8 as Trait>::Gat<'a>)`
    |
-   = note: expected opaque type `typeck::Opaque`
-              found raw pointer `*const (dyn FnOnce(()) + 'static)`
-   = help: consider constraining the associated type `<u8 as Trait>::Gat<'_>` to `()` or calling a method that returns `<u8 as Trait>::Gat<'_>`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+note: previous use here
+  --> $DIR/normalize-hidden-types.rs:44:9
+   |
+LL |         None
+   |         ^^^^
 
 error: concrete type differs from previous defining opaque type use
   --> $DIR/normalize-hidden-types.rs:52:25
@@ -52,4 +48,3 @@ LL |         None
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/self-referential-2.current.stderr

@@ -1,14 +0,0 @@
-error[E0277]: can't compare `i32` with `Foo`
-  --> $DIR/self-referential-2.rs:10:13
-   |
-LL | fn bar() -> Bar {
-   |             ^^^ no implementation for `i32 == Foo`
-LL |     42_i32
-   |     ------ return type was inferred to be `i32` here
-   |
-   = help: the trait `PartialEq<Foo>` is not implemented for `i32`
-   = help: the trait `PartialEq` is implemented for `i32`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0277`.

tests/ui/type-alias-impl-trait/self-referential-2.rs

@@ -1,14 +1,14 @@
 //@ revisions: current next
 //@ ignore-compare-mode-next-solver (explicit revisions)
 //@[next] compile-flags: -Znext-solver
-//@[next] check-pass
+//@ check-pass
 #![feature(type_alias_impl_trait)]
 
 type Foo = impl std::fmt::Debug;
 type Bar = impl PartialEq<Foo>;
 
 fn bar() -> Bar {
-    42_i32 //[current]~^ ERROR can't compare `i32` with `Foo`
+    42_i32
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/self-referential-3.rs

@@ -5,7 +5,7 @@
 type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>> + std::fmt::Debug;
 
 fn bar<'a, 'b>(i: &'a i32) -> Bar<'a, 'b> {
-    //~^ ERROR can't compare `&i32` with `Bar<'a, 'b>`
+    //~^ ERROR overflow normalizing the type alias `Bar<'a, 'b>`
     i
 }
 

tests/ui/type-alias-impl-trait/self-referential-3.stderr

@@ -1,15 +1,11 @@
-error[E0277]: can't compare `&i32` with `Bar<'a, 'b>`
+error[E0275]: overflow normalizing the type alias `Bar<'a, 'b>`
   --> $DIR/self-referential-3.rs:7:31
    |
 LL | fn bar<'a, 'b>(i: &'a i32) -> Bar<'a, 'b> {
-   |                               ^^^^^^^^^^^ no implementation for `&i32 == Bar<'a, 'b>`
-LL |
-LL |     i
-   |     - return type was inferred to be `&i32` here
+   |                               ^^^^^^^^^^^
    |
-   = help: the trait `PartialEq<Bar<'a, 'b>>` is not implemented for `&i32`
-   = help: the trait `PartialEq` is implemented for `i32`
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0275`.