Add a test for sibling items in trait impls

Author: oli-obk

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

@@ -0,0 +1,18 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo = impl std::fmt::Debug;
+
+trait Blah {
+    type Output;
+    fn method();
+}
+
+impl Blah for u32 {
+    type Output = Foo;
+    fn method() {
+        let x: Foo = 22_u32;
+        //~^ ERROR: opaque type constrained without being represented in the signature
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,10 @@
+error: opaque type constrained without being represented in the signature
+  --> $DIR/almost_in_scope.rs:13:22
+   |
+LL |     fn method() {
+   |     ----------- this item must mention the opaque type in its signature or where bounds
+LL |         let x: Foo = 22_u32;
+   |                      ^^^^^^
+
+error: aborting due to previous error
+