Rollup merge of #103488 - oli-obk:impl_trait_for_tait, r=lcnr

Allow opaque types in trait impl headers and rely on coherence to reject unsound cases

r? ````@lcnr````

fixes #99840

35 files changed, 292 insertions, 160 deletions

diff --git a/src/test/ui/coherence/coherence-with-closure.rs b/src/test/ui/coherence/coherence-with-closure.rs
index 6e3281d8508..5b6a62b24d4 100644
--- a/src/test/ui/coherence/coherence-with-closure.rs
+++ b/src/test/ui/coherence/coherence-with-closure.rs
@@ -8,7 +8,6 @@ fn defining_use() -> OpaqueClosure {
 struct Wrapper<T>(T);
 trait Trait {}
 impl Trait for Wrapper<OpaqueClosure> {}
-//~^ ERROR cannot implement trait on type alias impl trait
 impl<T: Sync> Trait for Wrapper<T> {}
 //~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
 
diff --git a/src/test/ui/coherence/coherence-with-closure.stderr b/src/test/ui/coherence/coherence-with-closure.stderr
index d2ca63fa146..431108e14d7 100644
--- a/src/test/ui/coherence/coherence-with-closure.stderr
+++ b/src/test/ui/coherence/coherence-with-closure.stderr
@@ -1,24 +1,11 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
-  --> $DIR/coherence-with-closure.rs:12:1
+  --> $DIR/coherence-with-closure.rs:11:1
    |
 LL | impl Trait for Wrapper<OpaqueClosure> {}
    | ------------------------------------- first implementation here
-LL |
 LL | impl<T: Sync> Trait for Wrapper<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueClosure>`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/coherence-with-closure.rs:10:24
-   |
-LL | impl Trait for Wrapper<OpaqueClosure> {}
-   |                        ^^^^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/coherence-with-closure.rs:3:22
-   |
-LL | type OpaqueClosure = impl Sized;
-   |                      ^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/coherence/coherence-with-generator.rs b/src/test/ui/coherence/coherence-with-generator.rs
index d34c391db9f..70665ba06f9 100644
--- a/src/test/ui/coherence/coherence-with-generator.rs
+++ b/src/test/ui/coherence/coherence-with-generator.rs
@@ -12,7 +12,6 @@ fn defining_use() -> OpaqueGenerator {
 struct Wrapper<T>(T);
 trait Trait {}
 impl Trait for Wrapper<OpaqueGenerator> {}
-//~^ ERROR cannot implement trait on type alias impl trait
 impl<T: Sync> Trait for Wrapper<T> {}
 //~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
 
diff --git a/src/test/ui/coherence/coherence-with-generator.stderr b/src/test/ui/coherence/coherence-with-generator.stderr
index 804bc1c3a6d..6d3be2e16c6 100644
--- a/src/test/ui/coherence/coherence-with-generator.stderr
+++ b/src/test/ui/coherence/coherence-with-generator.stderr
@@ -1,24 +1,11 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
-  --> $DIR/coherence-with-generator.rs:16:1
+  --> $DIR/coherence-with-generator.rs:15:1
    |
 LL | impl Trait for Wrapper<OpaqueGenerator> {}
    | --------------------------------------- first implementation here
-LL |
 LL | impl<T: Sync> Trait for Wrapper<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueGenerator>`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/coherence-with-generator.rs:14:24
-   |
-LL | impl Trait for Wrapper<OpaqueGenerator> {}
-   |                        ^^^^^^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/coherence-with-generator.rs:3:24
-   |
-LL | type OpaqueGenerator = impl Sized;
-   |                        ^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/auto-trait.rs b/src/test/ui/impl-trait/auto-trait.rs
index afa95645a27..35994e4a5ba 100644
--- a/src/test/ui/impl-trait/auto-trait.rs
+++ b/src/test/ui/impl-trait/auto-trait.rs
@@ -20,7 +20,6 @@ impl<T: Send> AnotherTrait for T {}
 // in the future.)
 impl AnotherTrait for D<OpaqueType> {
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
-    //~| ERROR cannot implement trait on type alias impl trait
 }
 
 fn main() {}
diff --git a/src/test/ui/impl-trait/auto-trait.stderr b/src/test/ui/impl-trait/auto-trait.stderr
index 5e10272b0db..81009413c9a 100644
--- a/src/test/ui/impl-trait/auto-trait.stderr
+++ b/src/test/ui/impl-trait/auto-trait.stderr
@@ -7,18 +7,6 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/auto-trait.rs:21:25
-   |
-LL | impl AnotherTrait for D<OpaqueType> {
-   |                         ^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/auto-trait.rs:7:19
-   |
-LL | type OpaqueType = impl OpaqueTrait;
-   |                   ^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/negative-reasoning.rs b/src/test/ui/impl-trait/negative-reasoning.rs
index da69bb349ae..70e24a3a9d0 100644
--- a/src/test/ui/impl-trait/negative-reasoning.rs
+++ b/src/test/ui/impl-trait/negative-reasoning.rs
@@ -18,7 +18,6 @@ impl<T: std::fmt::Debug> AnotherTrait for T {}
 // This is in error, because we cannot assume that `OpaqueType: !Debug`
 impl AnotherTrait for D<OpaqueType> {
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
-    //~| ERROR cannot implement trait on type alias impl trait
 }
 
 fn main() {}
diff --git a/src/test/ui/impl-trait/negative-reasoning.stderr b/src/test/ui/impl-trait/negative-reasoning.stderr
index 479b451855d..6b8cc9e7374 100644
--- a/src/test/ui/impl-trait/negative-reasoning.stderr
+++ b/src/test/ui/impl-trait/negative-reasoning.stderr
@@ -9,18 +9,6 @@ LL | impl AnotherTrait for D<OpaqueType> {
    |
    = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `OpaqueType` in future versions
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/negative-reasoning.rs:19:25
-   |
-LL | impl AnotherTrait for D<OpaqueType> {
-   |                         ^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/negative-reasoning.rs:7:19
-   |
-LL | type OpaqueType = impl OpaqueTrait;
-   |                   ^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs
index 621c4ea6e0d..af9dfe25bb4 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs
@@ -5,13 +5,13 @@ type Foo = impl PartialEq<(Foo, i32)>;
 struct Bar;
 
 impl PartialEq<(Foo, i32)> for Bar {
-//~^ ERROR cannot implement trait on type alias impl trait
     fn eq(&self, _other: &(Foo, i32)) -> bool {
         true
     }
 }
 
 fn foo() -> Foo {
+    //~^ ERROR can't compare `Bar` with `(Bar, i32)`
     Bar
 }
 
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr
index 2ef1697ba34..7b63a3d0b9f 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr
@@ -1,14 +1,15 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:7:17
+error[E0277]: can't compare `Bar` with `(Bar, i32)`
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:13:13
    |
-LL | impl PartialEq<(Foo, i32)> for Bar {
-   |                 ^^^
+LL | fn foo() -> Foo {
+   |             ^^^ no implementation for `Bar == (Bar, i32)`
+LL |
+LL |     Bar
+   |     --- return type was inferred to be `Bar` here
    |
-note: type alias impl trait defined here
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:3:12
-   |
-LL | type Foo = impl PartialEq<(Foo, i32)>;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: the trait `PartialEq<(Bar, i32)>` is not implemented for `Bar`
+   = help: the trait `PartialEq<(Foo, i32)>` is implemented for `Bar`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs
index df7966f00e1..91f1ed48133 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs
@@ -2,11 +2,13 @@
 
 mod a {
     type Foo = impl PartialEq<(Foo, i32)>;
+    //~^ ERROR: unconstrained opaque type
 
     struct Bar;
 
     impl PartialEq<(Bar, i32)> for Bar {
         fn eq(&self, _other: &(Foo, i32)) -> bool {
+            //~^ ERROR: `eq` has an incompatible type for trait
             true
         }
     }
@@ -14,12 +16,13 @@ mod a {
 
 mod b {
     type Foo = impl PartialEq<(Foo, i32)>;
+    //~^ ERROR: unconstrained opaque type
 
     struct Bar;
 
     impl PartialEq<(Foo, i32)> for Bar {
-        //~^ ERROR cannot implement trait on type alias impl trait
         fn eq(&self, _other: &(Bar, i32)) -> bool {
+            //~^ ERROR: `eq` has an incompatible type for trait
             true
         }
     }
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr
index 6cd63dcf81c..3dda5761ada 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr
@@ -1,14 +1,49 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:20:21
+error: unconstrained opaque type
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:4:16
    |
-LL |     impl PartialEq<(Foo, i32)> for Bar {
-   |                     ^^^
+LL |     type Foo = impl PartialEq<(Foo, i32)>;
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error[E0053]: method `eq` has an incompatible type for trait
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:10:30
+   |
+LL |     type Foo = impl PartialEq<(Foo, i32)>;
+   |                -------------------------- the found opaque type
+...
+LL |         fn eq(&self, _other: &(Foo, i32)) -> bool {
+   |                              ^^^^^^^^^^^
+   |                              |
+   |                              expected struct `a::Bar`, found opaque type
+   |                              help: change the parameter type to match the trait: `&(a::Bar, i32)`
    |
-note: type alias impl trait defined here
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:16:16
+   = note: expected fn pointer `fn(&a::Bar, &(a::Bar, i32)) -> _`
+              found fn pointer `fn(&a::Bar, &(a::Foo, i32)) -> _`
+
+error: unconstrained opaque type
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:18:16
    |
 LL |     type Foo = impl PartialEq<(Foo, i32)>;
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error[E0053]: method `eq` has an incompatible type for trait
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:24:30
+   |
+LL |     type Foo = impl PartialEq<(Foo, i32)>;
+   |                -------------------------- the expected opaque type
+...
+LL |         fn eq(&self, _other: &(Bar, i32)) -> bool {
+   |                              ^^^^^^^^^^^
+   |                              |
+   |                              expected opaque type, found struct `b::Bar`
+   |                              help: change the parameter type to match the trait: `&(b::Foo, i32)`
+   |
+   = note: expected fn pointer `fn(&b::Bar, &(b::Foo, i32)) -> _`
+              found fn pointer `fn(&b::Bar, &(b::Bar, i32)) -> _`
 
-error: aborting due to previous error
+error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0053`.
diff --git a/src/test/ui/traits/alias/issue-83613.rs b/src/test/ui/traits/alias/issue-83613.rs
index 04320e72076..2462e703a71 100644
--- a/src/test/ui/traits/alias/issue-83613.rs
+++ b/src/test/ui/traits/alias/issue-83613.rs
@@ -9,5 +9,4 @@ trait AnotherTrait {}
 impl<T: Send> AnotherTrait for T {}
 impl AnotherTrait for OpaqueType {}
 //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `OpaqueType`
-//~| ERROR cannot implement trait on type alias impl trait
 fn main() {}
diff --git a/src/test/ui/traits/alias/issue-83613.stderr b/src/test/ui/traits/alias/issue-83613.stderr
index b9d93160192..a78294da6c1 100644
--- a/src/test/ui/traits/alias/issue-83613.stderr
+++ b/src/test/ui/traits/alias/issue-83613.stderr
@@ -6,18 +6,6 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for OpaqueType {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `OpaqueType`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-83613.rs:10:23
-   |
-LL | impl AnotherTrait for OpaqueType {}
-   |                       ^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-83613.rs:4:19
-   |
-LL | type OpaqueType = impl OpaqueTrait;
-   |                   ^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/coherence.rs b/src/test/ui/type-alias-impl-trait/coherence.rs
index 98ac215ad6c..077a31494a9 100644
--- a/src/test/ui/type-alias-impl-trait/coherence.rs
+++ b/src/test/ui/type-alias-impl-trait/coherence.rs
@@ -12,6 +12,6 @@ fn use_alias<T>(val: T) -> AliasOfForeignType<T> {
 }
 
 impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
-//~^ ERROR cannot implement trait on type alias impl trait
+//~^ ERROR only traits defined in the current crate can be implemented for arbitrary types
 
 fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/coherence.stderr b/src/test/ui/type-alias-impl-trait/coherence.stderr
index 3ce25d94f6e..c923eb08ab3 100644
--- a/src/test/ui/type-alias-impl-trait/coherence.stderr
+++ b/src/test/ui/type-alias-impl-trait/coherence.stderr
@@ -1,14 +1,14 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/coherence.rs:14:41
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence.rs:14:1
    |
 LL | impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
-   |                                         ^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------------
+   | |                                       |
+   | |                                       `AliasOfForeignType<T>` is not defined in the current crate
+   | impl doesn't use only types from inside the current crate
    |
-note: type alias impl trait defined here
-  --> $DIR/coherence.rs:9:30
-   |
-LL | type AliasOfForeignType<T> = impl LocalTrait;
-   |                              ^^^^^^^^^^^^^^^
+   = note: define and implement a trait or new type instead
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0117`.
diff --git a/src/test/ui/type-alias-impl-trait/coherence_generalization.rs b/src/test/ui/type-alias-impl-trait/coherence_generalization.rs
new file mode 100644
index 00000000000..5c9ad9498b6
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/coherence_generalization.rs
@@ -0,0 +1,13 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+trait Trait {}
+type Opaque<T> = impl Sized;
+fn foo<T>() -> Opaque<T> {
+    ()
+}
+
+impl<T, V> Trait for (T, V, V, u32) {}
+impl<U, V> Trait for (Opaque<U>, V, i32, V) {}
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_generic_tait.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_generic_tait.rs
new file mode 100644
index 00000000000..0efbd1c2bd5
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_generic_tait.rs
@@ -0,0 +1,23 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+trait Foo {
+    type Assoc;
+}
+
+impl Foo for i32 {
+    type Assoc = u32;
+}
+type ImplTrait = impl Sized;
+fn constrain() -> ImplTrait {
+    1u64
+}
+impl Foo for i64 {
+    type Assoc = ImplTrait;
+}
+
+trait Bar<T> {}
+
+impl<T: Foo> Bar<<T as Foo>::Assoc> for T {}
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.rs
new file mode 100644
index 00000000000..3f1a9d12b44
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.rs
@@ -0,0 +1,33 @@
+#![feature(type_alias_impl_trait)]
+
+trait Foo {}
+impl Foo for () {}
+impl Foo for i32 {}
+
+type Bar<T: Foo> = impl std::fmt::Debug;
+fn defining_use<T: Foo>() -> Bar<T> {
+    42
+}
+
+trait Bop {}
+
+impl Bop for Bar<()> {}
+
+// If the hidden type is the same, this is effectively a second impl for the same type.
+impl Bop for Bar<i32> {}
+//~^ ERROR conflicting implementations
+
+type Barr = impl std::fmt::Debug;
+fn defining_use2() -> Barr {
+    42
+}
+
+// Even completely different opaque types must conflict.
+impl Bop for Barr {}
+//~^ ERROR conflicting implementations
+
+// And obviously the hidden type must conflict, too.
+impl Bop for i32 {}
+//~^ ERROR conflicting implementations
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr
new file mode 100644
index 00000000000..aaf75cc3db9
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr
@@ -0,0 +1,30 @@
+error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
+  --> $DIR/impl_trait_for_same_tait.rs:17:1
+   |
+LL | impl Bop for Bar<()> {}
+   | -------------------- first implementation here
+...
+LL | impl Bop for Bar<i32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+
+error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
+  --> $DIR/impl_trait_for_same_tait.rs:26:1
+   |
+LL | impl Bop for Bar<()> {}
+   | -------------------- first implementation here
+...
+LL | impl Bop for Barr {}
+   | ^^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+
+error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
+  --> $DIR/impl_trait_for_same_tait.rs:30:1
+   |
+LL | impl Bop for Bar<()> {}
+   | -------------------- first implementation here
+...
+LL | impl Bop for i32 {}
+   | ^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs
new file mode 100644
index 00000000000..9f32c5d888b
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs
@@ -0,0 +1,21 @@
+// compile-flags: --crate-type=lib
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+type Alias = impl Sized;
+
+fn constrain() -> Alias {
+    1i32
+}
+
+trait HideIt {
+    type Assoc;
+}
+
+impl HideIt for () {
+    type Assoc = Alias;
+}
+
+pub trait Yay {}
+
+impl Yay for <() as HideIt>::Assoc {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs
new file mode 100644
index 00000000000..8ec20acef4d
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs
@@ -0,0 +1,19 @@
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+pub trait Yay { }
+impl Yay for Foo { }
+
+fn foo() {
+    is_yay::<u32>();   //~ ERROR: the trait bound `u32: Yay` is not satisfied
+    is_debug::<u32>(); // OK
+    is_yay::<Foo>();   // OK
+    is_debug::<Foo>(); // OK
+}
+
+fn is_yay<T: Yay>() { }
+fn is_debug<T: Debug>() { }
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr
new file mode 100644
index 00000000000..1c83105a18a
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr
@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `u32: Yay` is not satisfied
+  --> $DIR/impl_trait_for_tait_bound.rs:10:14
+   |
+LL |     is_yay::<u32>();
+   |              ^^^ the trait `Yay` is not implemented for `u32`
+   |
+   = help: the trait `Yay` is implemented for `Foo`
+note: required by a bound in `is_yay`
+  --> $DIR/impl_trait_for_tait_bound.rs:16:14
+   |
+LL | fn is_yay<T: Yay>() { }
+   |              ^^^ required by this bound in `is_yay`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs
new file mode 100644
index 00000000000..a4b8c2d190d
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs
@@ -0,0 +1,16 @@
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+
+pub trait Yay { }
+impl Yay for u32 { }
+
+fn foo() {
+    is_yay::<Foo>(); //~ ERROR: the trait bound `Foo: Yay` is not satisfied
+}
+
+fn is_yay<T: Yay>() { }
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr
new file mode 100644
index 00000000000..a6440f02c27
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr
@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `Foo: Yay` is not satisfied
+  --> $DIR/impl_trait_for_tait_bound2.rs:11:14
+   |
+LL |     is_yay::<Foo>();
+   |              ^^^ the trait `Yay` is not implemented for `Foo`
+   |
+   = help: the trait `Yay` is implemented for `u32`
+note: required by a bound in `is_yay`
+  --> $DIR/impl_trait_for_tait_bound2.rs:14:14
+   |
+LL | fn is_yay<T: Yay>() { }
+   |              ^^^ required by this bound in `is_yay`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/issue-65384.rs b/src/test/ui/type-alias-impl-trait/issue-65384.rs
index 9a119c4d2e0..9a9b2269f80 100644
--- a/src/test/ui/type-alias-impl-trait/issue-65384.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-65384.rs
@@ -8,7 +8,7 @@ impl MyTrait for () {}
 type Bar = impl MyTrait;
 
 impl MyTrait for Bar {}
-//~^ ERROR: cannot implement trait on type alias impl trait
+//~^ ERROR: conflicting implementations of trait `MyTrait` for type `()`
 
 fn bazr() -> Bar { }
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-65384.stderr b/src/test/ui/type-alias-impl-trait/issue-65384.stderr
index 41bcea27e1f..f6692ae3207 100644
--- a/src/test/ui/type-alias-impl-trait/issue-65384.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-65384.stderr
@@ -1,14 +1,12 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-65384.rs:10:18
+error[E0119]: conflicting implementations of trait `MyTrait` for type `()`
+  --> $DIR/issue-65384.rs:10:1
    |
+LL | impl MyTrait for () {}
+   | ------------------- first implementation here
+...
 LL | impl MyTrait for Bar {}
-   |                  ^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-65384.rs:8:12
-   |
-LL | type Bar = impl MyTrait;
-   |            ^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs b/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs
index fb56cc54d63..b97e444c6d0 100644
--- a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs
@@ -1,6 +1,8 @@
 // Regression test for issue #76202
 // Tests that we don't ICE when we have a trait impl on a TAIT.
 
+// check-pass
+
 #![feature(type_alias_impl_trait)]
 
 trait Dummy {}
@@ -14,7 +16,12 @@ trait Test {
 }
 
 impl Test for F {
-    //~^ ERROR cannot implement trait
+    fn test(self) {}
+}
+
+// Ok because `i32` does not implement `Dummy`,
+// so it can't possibly be the hidden type of `F`.
+impl Test for i32 {
     fn test(self) {}
 }
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr b/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr
deleted file mode 100644
index 2d4a6854a92..00000000000
--- a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-76202-trait-impl-for-tait.rs:16:15
-   |
-LL | impl Test for F {
-   |               ^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-76202-trait-impl-for-tait.rs:9:10
-   |
-LL | type F = impl Dummy;
-   |          ^^^^^^^^^^
-
-error: aborting due to previous error
-
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
index fa25d8f762e..2ba4befea2a 100644
--- a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
@@ -1,6 +1,8 @@
 // Regression test for issues #84660 and #86411: both are variations on #76202.
 // Tests that we don't ICE when we have an opaque type appearing anywhere in an impl header.
 
+// check-pass
+
 #![feature(type_alias_impl_trait)]
 
 trait Foo {}
@@ -12,7 +14,7 @@ trait TraitArg<T> {
     fn f();
 }
 
-impl TraitArg<Bar> for () { //~ ERROR cannot implement trait
+impl TraitArg<Bar> for () {
     fn f() {
         println!("ho");
     }
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr
deleted file mode 100644
index bb70d07be59..00000000000
--- a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-84660-trait-impl-for-tait.rs:15:15
-   |
-LL | impl TraitArg<Bar> for () {
-   |               ^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-84660-trait-impl-for-tait.rs:8:12
-   |
-LL | type Bar = impl Foo;
-   |            ^^^^^^^^
-
-error: aborting due to previous error
-
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
index f12d1b6d953..48d4b0c96ff 100644
--- a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
@@ -13,14 +13,14 @@ trait Trait<T, In> {
     fn convert(i: In) -> Self::Out;
 }
 
-impl<In, Out> Trait<Bar, In> for Out { //~ ERROR cannot implement trait
+impl<In, Out> Trait<Bar, In> for Out {
     type Out = Out;
     fn convert(_i: In) -> Self::Out {
         unreachable!();
     }
 }
 
-impl<In, Out> Trait<(), In> for Out {
+impl<In, Out> Trait<(), In> for Out { //~ ERROR conflicting implementations of trait `Trait<Bar, _>`
     type Out = In;
     fn convert(i: In) -> Self::Out {
         i
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
index f2d600fb46c..6a75e1bd2c0 100644
--- a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
@@ -1,14 +1,12 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-84660-unsoundness.rs:16:21
+error[E0119]: conflicting implementations of trait `Trait<Bar, _>`
+  --> $DIR/issue-84660-unsoundness.rs:23:1
    |
 LL | impl<In, Out> Trait<Bar, In> for Out {
-   |                     ^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-84660-unsoundness.rs:8:12
-   |
-LL | type Bar = impl Foo;
-   |            ^^^^^^^^
+   | ------------------------------------ first implementation here
+...
+LL | impl<In, Out> Trait<(), In> for Out {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs
index ebf3a99bbf9..b0ebdd1bfab 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs
@@ -4,11 +4,11 @@
 use std::fmt::Debug;
 
 type FooX = impl Debug;
+//~^ ERROR unconstrained opaque type
 
 trait Foo<A> { }
 
 impl Foo<FooX> for () { }
-//~^ cannot implement trait on type alias impl trait
 
 fn foo() -> impl Foo<FooX> {
     ()
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr
index 4a3fb16733e..b1d947a9ccf 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr
@@ -1,14 +1,10 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/nested-tait-inference3.rs:10:10
-   |
-LL | impl Foo<FooX> for () { }
-   |          ^^^^
-   |
-note: type alias impl trait defined here
+error: unconstrained opaque type
   --> $DIR/nested-tait-inference3.rs:6:13
    |
 LL | type FooX = impl Debug;
    |             ^^^^^^^^^^
+   |
+   = note: `FooX` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error