add normalization test with defaulted type param with diverging alias

Another example from rust-lang/trait-system-refactor-initiative#139
and which was fixed in 1.89 `nightly-2025-06-18`.

Author: lqd

tests/ui/traits/normalize-diverging-alias-in-defaulted-type-parameter.current.stderr

@@ -0,0 +1,9 @@
+error[E0275]: overflow evaluating the requirement `<u8 as Trait>::Diverges<u8> == _`
+  --> $DIR/normalize-diverging-alias-in-defaulted-type-parameter.rs:17:12
+   |
+LL | struct Bar<T: ?Sized = <u8 as Trait>::Diverges<u8>>(Box<T>);
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/normalize-diverging-alias-in-defaulted-type-parameter.next.stderr

@@ -0,0 +1,9 @@
+error[E0271]: type mismatch resolving `<u8 as Trait>::Diverges<u8> normalizes-to _`
+  --> $DIR/normalize-diverging-alias-in-defaulted-type-parameter.rs:17:12
+   |
+LL | struct Bar<T: ?Sized = <u8 as Trait>::Diverges<u8>>(Box<T>);
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0271`.

tests/ui/traits/normalize-diverging-alias-in-defaulted-type-parameter.rs

@@ -0,0 +1,21 @@
+// Ensure that defaulted type parameters whose default contains diverging aliases are properly
+// caught with both solvers.
+// MCVE from https://github.com/rust-lang/trait-system-refactor-initiative/issues/139#issuecomment-2704576249.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@ [next] compile-flags: -Znext-solver
+
+trait Trait {
+    type Diverges<D: Trait>;
+}
+
+impl<T> Trait for T {
+    type Diverges<D: Trait> = D::Diverges<D>;
+}
+
+struct Bar<T: ?Sized = <u8 as Trait>::Diverges<u8>>(Box<T>);
+//[current]~^ ERROR: overflow evaluating the requirement `<u8 as Trait>::Diverges<u8> == _`
+//[next]~^^ ERROR: type mismatch resolving `<u8 as Trait>::Diverges<u8> normalizes-to _`
+
+fn main() {}