Make the test actually show the problematic case

src/test/ui/type-alias-impl-trait/implied_lifetime_wf_check.rs

@@ -1,18 +1,27 @@
-trait Mirror<'a> {
+#![feature(type_alias_impl_trait)]
-    type Item;
+
+// known-bug: #99840
+// this should not compile
+// check-pass
+
+type Alias = impl Sized;
+
+fn constrain() -> Alias {
+    1i32
 }
 
-impl<'a, T> Mirror<'a> for T {
+trait HideIt {
-    type Item = T;
+    type Assoc;
 }
 
-trait AnotherTrait {
+impl HideIt for () {
-    type Blah;
+    type Assoc = Alias;
 }
 
-impl<'a> AnotherTrait for <u32 as Mirror<'a>>::Item {
+pub trait Yay {}
-    //~^ ERROR: the lifetime parameter `'a` is not constrained
+
-    type Blah = &'a u32;
+impl Yay for <() as HideIt>::Assoc {}
-}
+// impl Yay for i32 {} // this already errors
+// impl Yay for u32 {} // this also already errors
 
 fn main() {}

src/test/ui/type-alias-impl-trait/implied_lifetime_wf_check.stderr

@@ -1,9 +0,0 @@
-error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
-  --> $DIR/implied_lifetime_wf_check.rs:13:6
-   |
-LL | impl<'a> AnotherTrait for <u32 as Mirror<'a>>::Item {
-   |      ^^ unconstrained lifetime parameter
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0207`.