coinductive cycle leak check test

tests/ui/traits/new-solver/leak-check-coinductive-cycle.rs

@@ -0,0 +1,33 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+#![feature(rustc_attrs)]
+
+#[rustc_coinductive]
+trait Trait<T> {}
+impl<'a, 'b, T> Trait<T> for (&'a (), &'b ())
+where
+    'b: 'a,
+    &'a (): Trait<T>,
+{}
+
+impl Trait<i32> for &'static () {}
+impl<'a> Trait<u32> for &'a ()
+where
+    for<'b> (&'a (), &'b ()): Trait<u32>,
+{}
+
+
+fn impls_trait<T: Trait<U>, U>() {}
+
+fn main() {
+    // This infers to `impls_trait::<(&'static (), &'static ()), i32>();`
+    //
+    // In the first attempt we have 2 candidates for `&'a (): Trait<_>`
+    // and we get ambiguity. The result is therefore ambiguity with a `'b: 'a`
+    // constraint. The next attempt then uses that provisional result when
+    // trying to apply `impl<'a> Trait<u32> for &'a ()`. This means we get a
+    // `for<'b> 'b: 'a` bound which fails the leak check. Because of this we
+    // end up with a single impl for `&'a (): Trait<_>` which infers `_` to `i32`
+    // and succeeds.
+    impls_trait::<(&(), &()), _>();
+}