Fix bugs in evaluating WellFormed predicates

- List the nestsed obligations in an order that works with the
  single pass used by evaluation
- Propagate recursion depth correctly

1 files changed, 39 insertions, 0 deletions

diff --git a/src/test/ui/impl-trait/wf-eval-order.rs b/src/test/ui/impl-trait/wf-eval-order.rs
new file mode 100644
index 00000000000..c7d6bb87096
--- /dev/null
+++ b/src/test/ui/impl-trait/wf-eval-order.rs
@@ -0,0 +1,39 @@
+// Check that we handle evaluating `wf` predicates correctly.
+
+// check-pass
+
+struct X<T: B>(T)
+where
+    T::V: Clone;
+
+fn hide<T>(t: T) -> impl Sized {
+    t
+}
+
+trait A {
+    type U;
+}
+
+impl<T> A for T {
+    type U = T;
+}
+
+trait B {
+    type V;
+}
+
+impl<S: A<U = T>, T> B for S {
+    type V = T;
+}
+
+fn main() {
+    // Evaluating `typeof(x): Sized` requires
+    //
+    // - `wf(typeof(x))` because we use a projection candidate.
+    // - `<i32 as B>::V: Clone` because that's a bound on the trait.
+    // - `<i32 as B>::V` normalizes to `_#1` where `<i32 as A>::U == _#1`
+    //
+    // This all works if we evaluate `<i32 as A>::U == _#1` before
+    // `<i32 as B>::V`, but we previously had the opposite order.
+    let x = hide(X(0));
+}