Check that we forbid nested items, but not nested closures

tests/ui/type-alias-impl-trait/itiat-allow-nested-closures.bad.stderr

@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+  --> $DIR/itiat-allow-nested-closures.rs:21:22
+   |
+LL |     type Assoc = impl Sized;
+   |                  ---------- the found opaque type
+...
+LL |         let _: i32 = closure();
+   |                ---   ^^^^^^^^^ expected `i32`, found opaque type
+   |                |
+   |                expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/itiat-allow-nested-closures.rs:22:9
+   |
+LL |     fn bar() -> Self::Assoc {
+   |                 ----------- expected `()` because of return type
+...
+LL |         1i32
+   |         ^^^^ expected `()`, found `i32`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/itiat-allow-nested-closures.rs

@@ -0,0 +1,26 @@
+#![feature(impl_trait_in_assoc_type)]
+
+// revisions: ok bad
+// [ok] check-pass
+
+trait Foo {
+    type Assoc;
+    fn bar() -> Self::Assoc;
+}
+
+impl Foo for () {
+    type Assoc = impl Sized;
+    fn bar() -> Self::Assoc {
+        let closure = || -> Self::Assoc {
+            #[cfg(ok)]
+            let x: Self::Assoc = 42_i32;
+            #[cfg(bad)]
+            let x: Self::Assoc = ();
+            x
+        };
+        let _: i32 = closure(); //[bad]~ ERROR mismatched types
+        1i32 //[bad]~ ERROR mismatched types
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/itiat-forbid-nested-items.rs

@@ -0,0 +1,20 @@
+#![feature(impl_trait_in_assoc_type)]
+
+trait Foo {
+    type Assoc;
+    fn bar() -> Self::Assoc;
+}
+
+impl Foo for () {
+    type Assoc = impl Sized;
+    fn bar() -> Self::Assoc {
+        fn foo() -> <() as Foo>::Assoc {
+            let x: <() as Foo>::Assoc = 42_i32; //~ ERROR mismatched types
+            x
+        };
+        let _: i32 = foo();
+        1i32
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/itiat-forbid-nested-items.stderr

@@ -0,0 +1,22 @@
+error[E0308]: mismatched types
+  --> $DIR/itiat-forbid-nested-items.rs:12:41
+   |
+LL |     type Assoc = impl Sized;
+   |                  ---------- the expected opaque type
+...
+LL |             let x: <() as Foo>::Assoc = 42_i32;
+   |                    ------------------   ^^^^^^ expected opaque type, found `i32`
+   |                    |
+   |                    expected due to this
+   |
+   = note: expected opaque type `<() as Foo>::Assoc`
+                     found type `i32`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/itiat-forbid-nested-items.rs:11:12
+   |
+LL |         fn foo() -> <() as Foo>::Assoc {
+   |            ^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.