Add test in higher ranked subtype

tests/ui/higher-ranked/structually-relate-aliases.rs

@@ -0,0 +1,17 @@
+// regression test for issue #121649.
+
+trait ToUnit<'a> {
+    type Unit;
+}
+
+trait Overlap<T> {}
+
+type Assoc<'a, T> = <T as ToUnit<'a>>::Unit;
+
+impl<T> Overlap<T> for T {}
+
+impl<T> Overlap<for<'a> fn(&'a (), Assoc<'a, T>)> for T {}
+//~^ ERROR 13:17: 13:49: the trait bound `for<'a> T: ToUnit<'a>` is not satisfied [E0277]
+//~| ERROR 13:36: 13:48: the trait bound `for<'a> T: ToUnit<'a>` is not satisfied [E0277]
+
+fn main() {}

tests/ui/higher-ranked/structually-relate-aliases.stderr

@@ -0,0 +1,27 @@
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [?1t, '^0.Named(DefId(0:15 ~ structually_relate_aliases[de75]::{impl#1}::'a), "'a")], def_id: DefId(0:5 ~ structually_relate_aliases[de75]::ToUnit::Unit) }
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: AliasTy { args: [?1t, !2_0.Named(DefId(0:15 ~ structually_relate_aliases[de75]::{impl#1}::'a), "'a")], def_id: DefId(0:5 ~ structually_relate_aliases[de75]::ToUnit::Unit) }
+error[E0277]: the trait bound `for<'a> T: ToUnit<'a>` is not satisfied
+  --> $DIR/structually-relate-aliases.rs:13:36
+   |
+LL | impl<T> Overlap<for<'a> fn(&'a (), Assoc<'a, T>)> for T {}
+   |                                    ^^^^^^^^^^^^ the trait `for<'a> ToUnit<'a>` is not implemented for `T`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | impl<T: for<'a> ToUnit<'a>> Overlap<for<'a> fn(&'a (), Assoc<'a, T>)> for T {}
+   |       ++++++++++++++++++++
+
+error[E0277]: the trait bound `for<'a> T: ToUnit<'a>` is not satisfied
+  --> $DIR/structually-relate-aliases.rs:13:17
+   |
+LL | impl<T> Overlap<for<'a> fn(&'a (), Assoc<'a, T>)> for T {}
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'a> ToUnit<'a>` is not implemented for `T`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | impl<T: for<'a> ToUnit<'a>> Overlap<for<'a> fn(&'a (), Assoc<'a, T>)> for T {}
+   |       ++++++++++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.