Add regression test for cross crate specialization query cycle

tests/ui/specialization/auxiliary/impls-cycle-90817.rs

@@ -0,0 +1,59 @@
+pub trait Reducible {
+    type Reduced;
+}
+
+pub struct Base;
+
+pub struct Higher;
+
+impl Reducible for Higher {
+    type Reduced = Base;
+}
+
+pub trait Reduce<R> {}
+
+impl<T> Reduce<Base> for T {}
+
+impl<T, R> Reduce<R> for T
+where
+    R: Reducible,
+    T: Reduce<R::Reduced>,
+{
+}
+
+pub fn reduce<R>(_: R)
+where
+    (): Reduce<R>,
+{
+}
+
+pub trait Change<T> {
+    type Changed;
+}
+
+impl<T> Change<T> for () {
+    type Changed = T;
+}
+
+pub trait Split {
+    type First;
+    type Rest;
+}
+
+pub trait Combine<T> {}
+
+impl<U> Combine<()> for U {}
+
+impl<T, U> Combine<U> for T
+where
+    U: Split,
+    Self: Change<U::First>,
+    <Self as Change<U::First>>::Changed: Combine<U::Rest>,
+{
+}
+
+pub fn combine<T>(_: T)
+where
+    (): Combine<T>,
+{
+}

tests/ui/specialization/cross-crate-impls-cycle-90817.rs

@@ -0,0 +1,12 @@
+//@ aux-build:impls-cycle-90817.rs
+//@ check-pass
+//! Regression test for <https://github.com/rust-lang/rust/issues/90817>.
+//! Checking whether these impls specialize one another used to hit a query
+//! cycle (E0391) when the impls live in another crate.
+
+extern crate impls_cycle_90817;
+
+fn main() {
+    impls_cycle_90817::reduce(impls_cycle_90817::Higher);
+    impls_cycle_90817::combine(());
+}