Require any function with a tait in its signature to actually constra…

…in a hidden type

example/issue-72793.rs

@@ -2,20 +2,23 @@
 
 #![feature(type_alias_impl_trait)]
 
-trait T {
-    type Item;
-}
+mod helper {
+    pub trait T {
+        type Item;
+    }
 
-type Alias<'a> = impl T<Item = &'a ()>;
+    pub type Alias<'a> = impl T<Item = &'a ()>;
 
-struct S;
-impl<'a> T for &'a S {
-    type Item = &'a ();
-}
+    struct S;
+    impl<'a> T for &'a S {
+        type Item = &'a ();
+    }
 
-fn filter_positive<'a>() -> Alias<'a> {
-    &S
+    pub fn filter_positive<'a>() -> Alias<'a> {
+        &S
+    }
 }
+use helper::*;
 
 fn with_positive(fun: impl Fn(Alias<'_>)) {
     fun(filter_positive());