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tests/ui/type-inference/regression-issue-81317.rs

@@ -1,4 +1,33 @@
 // Regression test for #81317: type can no longer be infered as of 1.49
+//
+// The problem is that the xor operator and the index.into() each have two candidate impls that could apply
+// { S as BitXor<S>, S as BitXor<&'a S> } for xor and
+// { T::I as Into<u64>, T::I as Into<S> } for index.into()
+// previously inference was able to infer that the only valid combination was
+// S as BitXor<S> and T::I as Into<S>
+//
+// after rust-lang/rust#73905 this is no longer infered
+//
+// the error message could be better e.g. when iv is unused or has an an explicitly specified type S
+// there is currently the following help message
+//
+// error[E0284]: type annotations needed
+//   --> src/main.rs:13:24
+//    |
+// 42 |     let iv = S ^ index.into();
+//    |                -       ^^^^
+//    |                |
+//    |                type must be known at this point
+//    |
+//    = note: cannot satisfy `<S as BitXor<_>>::Output == _`
+// help: try using a fully qualified path to specify the expected types
+//    |
+// 42 -     let iv = S ^ index.into();
+// 42 +     let iv = S ^ <<T as P>::I as Into<T>>::into(index);
+//
+// this is better as it's actually sufficent to fix the problem,
+// while just specifying the type of iv as currently suggested is insufficent
+//
 //@ check-fail
 
 use std::ops::BitXor;