add regression test

Author: lcnr

tests/ui/traits/next-solver/cycles/many-where-clauses-with-aliases-hang.rs

@@ -0,0 +1,44 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// A regression test for trait-system-refactor-initiative#210.
+//
+// Trying to prove `T: Trait<...>` ends up trying to apply all the where-clauses,
+// most of which require normalizing some `Alias<T, ...>`. This then requires us
+// to prove `T: Trait<...>` again.
+//
+// This results in a lot of solver cycles whose initial result differs from their
+// final result. Reevaluating all of them results in exponential blowup and hangs.
+//
+// With #144991 we now don't reevaluate cycle heads if their provisional value
+// didn't actually impact the final result, avoiding these reruns and allowing us
+// to compile this in less than a second.
+
+struct A;
+struct B;
+struct C;
+
+type Alias<T, U> = <T as Trait<U>>::Assoc;
+trait Trait<T> {
+    type Assoc;
+}
+
+fn foo<T>()
+where
+    T: Trait<A> + Trait<B> + Trait<C>,
+    T: Trait<Alias<T, A>>,
+    T: Trait<Alias<T, B>>,
+    T: Trait<Alias<T, C>>,
+    T: Trait<Alias<T, Alias<T, A>>>,
+    T: Trait<Alias<T, Alias<T, B>>>,
+    T: Trait<Alias<T, Alias<T, C>>>,
+    T: Trait<Alias<T, Alias<T, Alias<T, A>>>>,
+    T: Trait<Alias<T, Alias<T, Alias<T, B>>>>,
+    T: Trait<Alias<T, Alias<T, Alias<T, C>>>>,
+    T: Trait<Alias<T, Alias<T, Alias<T, Alias<T, A>>>>>,
+    T: Trait<Alias<T, Alias<T, Alias<T, Alias<T, B>>>>>,
+    T: Trait<Alias<T, Alias<T, Alias<T, Alias<T, C>>>>>,
+{
+}
+fn main() {}
+