[unconditional_recursion]: compare by types instead of DefIds

tests/ui/unconditional_recursion.rs

@@ -288,4 +288,39 @@ impl PartialEq for S15<'_> {
     }
 }
 
+mod issue12154 {
+    struct MyBox<T>(T);
+
+    impl<T> std::ops::Deref for MyBox<T> {
+        type Target = T;
+        fn deref(&self) -> &T {
+            &self.0
+        }
+    }
+
+    impl<T: PartialEq> PartialEq for MyBox<T> {
+        fn eq(&self, other: &Self) -> bool {
+            (**self).eq(&**other)
+        }
+    }
+
+    // Not necessarily related to the issue but another FP from the http crate that was fixed with it:
+    // https://docs.rs/http/latest/src/http/header/name.rs.html#1424
+    // We used to simply peel refs from the LHS and RHS, so we couldn't differentiate
+    // between `PartialEq<T> for &T` and `PartialEq<&T> for T` impls.
+    #[derive(PartialEq)]
+    struct HeaderName;
+    impl<'a> PartialEq<&'a HeaderName> for HeaderName {
+        fn eq(&self, other: &&'a HeaderName) -> bool {
+            *self == **other
+        }
+    }
+
+    impl<'a> PartialEq<HeaderName> for &'a HeaderName {
+        fn eq(&self, other: &HeaderName) -> bool {
+            *other == *self
+        }
+    }
+}
+
 fn main() {}