1 files changed, 69 insertions, 0 deletions
diff --git a/tests/ui/traits/solver-cycles/inductive-canonical-cycle.rs b/tests/ui/traits/solver-cycles/inductive-canonical-cycle.rs
new file mode 100644
index 000000000..5449f5f00
--- /dev/null
+++ b/tests/ui/traits/solver-cycles/inductive-canonical-cycle.rs
@@ -0,0 +1,69 @@
+// check-pass
+
+// This test checks that we're correctly dealing with inductive cycles
+// with canonical inference variables.
+
+trait Trait<T, U> {}
+
+trait IsNotU32 {}
+impl IsNotU32 for i32 {}
+impl<T: IsNotU32, U> Trait<T, U> for () // impl 1
+where
+    (): Trait<U, T>
+{}
+
+impl<T> Trait<u32, T> for () {} // impl 2
+
+// If we now check whether `(): Trait<?0, ?1>` holds this has to
+// result in ambiguity as both `for<T> (): Trait<u32, T>` and `(): Trait<i32, u32>`
+// applies. The remainder of this test asserts that.
+
+// If we were to error on inductive cycles with canonical inference variables
+// this would be wrong:
+
+// (): Trait<?0, ?1>
+//  - impl 1
+//      - ?0: IsNotU32 // ambig
+//      - (): Trait<?1, ?0> // canonical cycle -> err
+//      - ERR
+//  - impl 2
+//      - OK ?0 == u32
+//
+// Result: OK ?0 == u32.
+
+// (): Trait<i32, u32>
+//  - impl 1
+//      - i32: IsNotU32 // ok
+//      - (): Trait<u32, i32>
+//          - impl 1
+//              - u32: IsNotU32 // err
+//              - ERR
+//          - impl 2
+//              - OK
+//      - OK
+//  - impl 2 (trivial ERR)
+//
+// Result OK
+
+// This would mean that `(): Trait<?0, ?1>` is not complete,
+// which is unsound if we're in coherence.
+
+fn implements_trait<T, U>() -> (T, U)
+where
+    (): Trait<T, U>,
+{
+    todo!()
+}
+
+// A hack to only constrain the infer vars after first checking
+// the `(): Trait<_, _>`.
+trait Constrain<T> {}
+impl<T> Constrain<T> for  T {}
+fn constrain<T: Constrain<U>, U>(_: U) {}
+
+fn main() {
+    let (x, y) = implements_trait::<_, _>();
+
+    constrain::<i32, _>(x);
+    constrain::<u32, _>(y);
+}