Add some tests

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

@@ -0,0 +1,38 @@
+//! Check that we cannot instantiate a hidden type in the body
+//! of an assoc fn or const unless mentioned in the signature.
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo();
+    fn bar() -> Self::Assoc;
+}
+
+impl Trait for () {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() {
+        let x: Self::Assoc = 42; //~ ERROR: mismatched types
+    }
+    fn bar() -> Self::Assoc {
+        ""
+    }
+}
+
+trait Trait2: Sized {
+    type Assoc;
+    const FOO: ();
+    fn bar() -> Self::Assoc;
+}
+
+impl Trait2 for () {
+    type Assoc = impl std::fmt::Debug;
+    const FOO: () = {
+        let x: Self::Assoc = 42; //~ ERROR: mismatched types
+    };
+    fn bar() -> Self::Assoc {
+        ""
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait.stderr

@@ -0,0 +1,41 @@
+error[E0308]: mismatched types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:15:30
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     fn foo() {
+LL |         let x: Self::Assoc = 42;
+   |                -----------   ^^ expected opaque type, found integer
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Trait>::Assoc`
+                     found type `{integer}`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:14:8
+   |
+LL |     fn foo() {
+   |        ^^^
+
+error[E0308]: mismatched types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:31:30
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     const FOO: () = {
+LL |         let x: Self::Assoc = 42;
+   |                -----------   ^^ expected opaque type, found integer
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Trait2>::Assoc`
+                     found type `{integer}`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait.rs:30:11
+   |
+LL |     const FOO: () = {
+   |           ^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait2.rs

@@ -0,0 +1,22 @@
+//! Check that we cannot instantiate a hidden type from another assoc type.
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait: Sized {
+    type Assoc;
+    type Foo;
+    fn foo() -> Self::Assoc;
+}
+
+impl Trait for () {
+    type Assoc = impl std::fmt::Debug;
+    type Foo = [(); {
+        let x: Self::Assoc = 42; //~ ERROR: mismatched types
+        3
+    }];
+    fn foo() -> Self::Assoc {
+        ""
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait2.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/impl_trait_in_trait_defined_outside_trait2.rs:14:30
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     type Foo = [(); {
+LL |         let x: Self::Assoc = 42;
+   |                -----------   ^^ expected opaque type, found integer
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Trait>::Assoc`
+                     found type `{integer}`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/impl_trait_in_trait_defined_outside_trait3.rs

@@ -0,0 +1,38 @@
+//! Check that non-defining assoc items can use the opaque type
+//! opaquely.
+
+// check-pass
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo();
+    fn bar() -> Self::Assoc;
+}
+
+impl Trait for () {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() {
+        let x: Self::Assoc = Self::bar();
+    }
+    fn bar() -> Self::Assoc {
+        ""
+    }
+}
+
+trait Trait2: Sized {
+    type Assoc;
+    const FOO: ();
+    const BAR: Self::Assoc;
+}
+
+impl Trait2 for () {
+    type Assoc = impl Copy;
+    const FOO: () = {
+        let x: Self::Assoc = Self::BAR;
+    };
+    const BAR: Self::Assoc = "";
+}
+
+fn main() {}