Fix derived PartialOrd operators

The derived implementation of `partial_cmp` compares matching fields one
by one, stopping the computation when the result of a comparison is not
equal to `Some(Equal)`.

On the other hand the derived implementation for `lt`, `le`, `gt` and
`ge` continues the computation when the result of a field comparison is
`None`, consequently those operators are not transitive and inconsistent
with `partial_cmp`.

Fix the inconsistency by using the default implementation that fall-backs
to the `partial_cmp`. This also avoids creating very deeply nested
closures that were quite costly to compile.

1 files changed, 60 insertions, 0 deletions

diff --git a/src/test/ui/derives/derive-partial-ord.rs b/src/test/ui/derives/derive-partial-ord.rs
new file mode 100644
index 00000000000..9078a7ffa4f
--- /dev/null
+++ b/src/test/ui/derives/derive-partial-ord.rs
@@ -0,0 +1,60 @@
+// Checks that in a derived implementation of PartialOrd the lt, le, ge, gt methods are consistent
+// with partial_cmp. Also verifies that implementation is consistent with that for tuples.
+//
+// run-pass
+
+#[derive(PartialEq, PartialOrd)]
+struct P(f64, f64);
+
+fn main() {
+    let values: &[f64] = &[1.0, 2.0, f64::NAN];
+    for a in values {
+        for b in values {
+            for c in values {
+                for d in values {
+                    // Check impl for a tuple.
+                    check(&(*a, *b), &(*c, *d));
+
+                    // Check derived impl.
+                    check(&P(*a, *b), &P(*c, *d));
+
+                    // Check that impls agree with each other.
+                    assert_eq!(
+                        PartialOrd::partial_cmp(&(*a, *b), &(*c, *d)),
+                        PartialOrd::partial_cmp(&P(*a, *b), &P(*c, *d)),
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn check<T: PartialOrd>(a: &T, b: &T) {
+    use std::cmp::Ordering::*;
+    match PartialOrd::partial_cmp(a, b) {
+        None => {
+            assert!(!(a < b));
+            assert!(!(a <= b));
+            assert!(!(a > b));
+            assert!(!(a >= b));
+        }
+        Some(Equal) => {
+            assert!(!(a < b));
+            assert!(a <= b);
+            assert!(!(a > b));
+            assert!(a >= b);
+        }
+        Some(Less) => {
+            assert!(a < b);
+            assert!(a <= b);
+            assert!(!(a > b));
+            assert!(!(a >= b));
+        }
+        Some(Greater) => {
+            assert!(!(a < b));
+            assert!(!(a <= b));
+            assert!(a > b);
+            assert!(a >= b);
+        }
+    }
+}