1 files changed, 63 insertions, 0 deletions
diff --git a/src/test/ui/generic-associated-types/projection-bound-cycle.rs b/src/test/ui/generic-associated-types/projection-bound-cycle.rs
new file mode 100644
index 000000000..6564a3608
--- /dev/null
+++ b/src/test/ui/generic-associated-types/projection-bound-cycle.rs
@@ -0,0 +1,63 @@
+// Test case from Chalk.
+// Make sure that we make sure that we don't allow arbitrary bounds to be
+// proven when a bound and a where clause of an associated type are the same.
+
+#![feature(generic_associated_types)]
+#![feature(trivial_bounds)]
+
+trait Print {
+    fn print();
+}
+
+trait Foo {
+    type Item: Sized where <Self as Foo>::Item: Sized;
+}
+
+struct Number { }
+
+impl Foo for Number {
+    // Well-formedness checks require that the following
+    // goal is true:
+    // ```
+    // if (str: Sized) { # if the where clauses hold
+    //     str: Sized # then the bound on the associated type hold
+    // }
+    // ```
+    // which it is :)
+    type Item = str where str: Sized;
+}
+
+struct OnlySized<T> where T: Sized { f: T }
+impl<T> Print for OnlySized<T> {
+    fn print() {
+        println!("{}", std::mem::size_of::<T>());
+    }
+}
+
+trait Bar {
+    type Assoc: Print;
+}
+
+impl<T> Bar for T where T: Foo {
+    // This is not ok, we need to prove `wf(<T as Foo>::Item)`, which requires
+    // knowing that `<T as Foo>::Item: Sized` to satisfy the where clause. We
+    // can use the bound on `Foo::Item` for this, but that requires
+    // `wf(<T as Foo>::Item)`, which is an invalid cycle.
+    type Assoc = OnlySized<<T as Foo>::Item>;
+    //~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: Sized`
+}
+
+fn foo<T: Print>() {
+    T::print() // oops, in fact `T = OnlySized<str>` which is ill-formed
+}
+
+fn bar<T: Bar>() {
+    // we have `FromEnv(T: Bar)` hence
+    // `<T as Bar>::Assoc` is well-formed and
+    // `Implemented(<T as Bar>::Assoc: Print)` hold
+    foo::<<T as Bar>::Assoc>()
+}
+
+fn main() {
+    bar::<Number>()
+}