Add test for #115019

tests/ui/associated-type-bounds/overlaping-bound-suggestion.rs

+#![allow(bare_trait_objects)]
+#![feature(associated_type_bounds)]
+trait Item {
+    type Core;
+}
+pub struct Flatten<I> {
+    inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>::Core,
+    //~^ ERROR E0191
+    //~| ERROR E0223
+}
+
+fn main() {}

tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr

+error[E0191]: the value of the associated types `IntoIter` (from trait `IntoIterator`), `IntoIter` (from trait `IntoIterator`), `Item` (from trait `IntoIterator`), `Item` (from trait `IntoIterator`) must be specified
+  --> $DIR/overlaping-bound-suggestion.rs:7:13
+   |
+LL |     inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>::Core,
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             |                  |
+   |             |                  associated types `Item`, `IntoIter` must be specified
+   |             associated types `Item`, `IntoIter` must be specified
+   |
+help: specify the associated types
+   |
+LL |     inner: <IntoIterator<Item: IntoIterator<Item: >, Item = Type, IntoIter = Type>IntoIterator<Item: , Item = Type, IntoIter = Type>>::IntoIterator as Item>::Core,
+   |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error[E0223]: ambiguous associated type
+  --> $DIR/overlaping-bound-suggestion.rs:7:13
+   |
+LL |     inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>::Core,
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: if there were a trait named `Example` with associated type `IntoIterator` implemented for `(dyn IntoIterator + 'static)`, you could use the fully-qualified path
+   |
+LL |     inner: <<(dyn IntoIterator + 'static) as Example>::IntoIterator as Item>::Core,
+   |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0191, E0223.
+For more information about an error, try `rustc --explain E0191`.