Add test for #101844

Author: jackh726

src/test/ui/mir/issue-101844.rs

@@ -0,0 +1,73 @@
+// check-pass
+
+pub trait FirstTrait {
+    type Item;
+    type Extra: Extra<(), Error = Self::Item>;
+}
+
+trait SecondTrait {
+    type Item2;
+}
+
+trait ThirdTrait: SecondTrait {
+    type Item3;
+}
+
+trait FourthTrait {
+    type Item4;
+}
+
+impl<First> SecondTrait for First
+where
+    First: FirstTrait,
+{
+    type Item2 = First::Extra;
+}
+
+impl<Second, T> ThirdTrait for Second
+where
+    Second: SecondTrait<Item2 = T>,
+{
+    type Item3 = T;
+}
+
+impl<S, Third: ?Sized> FourthTrait for Third
+where
+    Third: ThirdTrait<Item3 = S>,
+{
+    type Item4 = S;
+}
+
+pub trait Extra<Request> {
+    type Error;
+}
+
+struct ImplShoulExist<D, Req> {
+    _gen: (D, Req),
+}
+
+impl<D, Req> ImplShoulExist<D, Req>
+where
+    D: FourthTrait,
+    D::Item4: Extra<Req>,
+    <D::Item4 as Extra<Req>>::Error: Into<()>,
+{
+    fn access_fn(_: D) {
+        todo!()
+    }
+}
+
+impl<D, Req> Extra<Req> for ImplShoulExist<D, Req> {
+    type Error = ();
+}
+
+pub fn broken<MS>(ms: MS)
+where
+    MS: FirstTrait,
+    MS::Item: Into<()>,
+{
+    // Error: Apparently Balance::new doesn't exist during MIR validation
+    let _ = ImplShoulExist::<MS, ()>::access_fn(ms);
+}
+
+fn main() {}