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diff --git a/src/tools/clippy/tests/ui/neg_cmp_op_on_partial_ord.rs b/src/tools/clippy/tests/ui/neg_cmp_op_on_partial_ord.rs
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+++ b/src/tools/clippy/tests/ui/neg_cmp_op_on_partial_ord.rs
@@ -0,0 +1,62 @@
+//! This test case utilizes `f64` an easy example for `PartialOrd` only types
+//! but the lint itself actually validates any expression where the left
+//! operand implements `PartialOrd` but not `Ord`.
+
+use std::cmp::Ordering;
+
+#[allow(clippy::unnested_or_patterns, clippy::match_like_matches_macro)]
+#[warn(clippy::neg_cmp_op_on_partial_ord)]
+fn main() {
+    let a_value = 1.0;
+    let another_value = 7.0;
+
+    // --- Bad ---
+
+    // Not Less but potentially Greater, Equal or Uncomparable.
+    let _not_less = !(a_value < another_value);
+
+    // Not Less or Equal but potentially Greater or Uncomparable.
+    let _not_less_or_equal = !(a_value <= another_value);
+
+    // Not Greater but potentially Less, Equal or Uncomparable.
+    let _not_greater = !(a_value > another_value);
+
+    // Not Greater or Equal but potentially Less or Uncomparable.
+    let _not_greater_or_equal = !(a_value >= another_value);
+
+    // --- Good ---
+
+    let _not_less = match a_value.partial_cmp(&another_value) {
+        None | Some(Ordering::Greater) | Some(Ordering::Equal) => true,
+        _ => false,
+    };
+    let _not_less_or_equal = match a_value.partial_cmp(&another_value) {
+        None | Some(Ordering::Greater) => true,
+        _ => false,
+    };
+    let _not_greater = match a_value.partial_cmp(&another_value) {
+        None | Some(Ordering::Less) | Some(Ordering::Equal) => true,
+        _ => false,
+    };
+    let _not_greater_or_equal = match a_value.partial_cmp(&another_value) {
+        None | Some(Ordering::Less) => true,
+        _ => false,
+    };
+
+    // --- Should not trigger ---
+
+    let _ = a_value < another_value;
+    let _ = a_value <= another_value;
+    let _ = a_value > another_value;
+    let _ = a_value >= another_value;
+
+    // --- regression tests ---
+
+    // Issue 2856: False positive on assert!()
+    //
+    // The macro always negates the result of the given comparison in its
+    // internal check which automatically triggered the lint. As it's an
+    // external macro there was no chance to do anything about it which led
+    // to an exempting of all external macros.
+    assert!(a_value < another_value);
+}