Use the same hash for +0 and -0.

src/number.rs

@@ -55,7 +55,14 @@ impl core::hash::Hash for N {
             N::NegInt(i) => i.hash(h),
             N::Float(f) => {
                 // Using `f64::to_bits` here is fine since any float values are never `NaN`.
-                f.to_bits().hash(h);
+                if *f == 0.0f64 {
+                    // The IEEE 754 standard has two representations for zero, +0 and -0,
+                    // such that +0 == -0.
+                    // In both cases we use the +0 hash so that hash(+0) == hash(-0).
+                    0.0f64.to_bits().hash(h);
+                } else {
+                    f.to_bits().hash(h);
+                }
             }
         }
     }

tests/test.rs

@@ -2284,3 +2284,19 @@ fn test_value_into_deserializer() {
     let outer = Outer::deserialize(map.into_deserializer()).unwrap();
     assert_eq!(outer.inner.string, "Hello World");
 }
+
+#[test]
+fn hash_positive_and_negative_zero() {
+    use std::collections::hash_map::DefaultHasher;
+    use std::hash::{Hash, Hasher};
+
+    fn hash(obj: impl Hash) -> u64 {
+        let mut hasher = DefaultHasher::new();
+        obj.hash(&mut hasher);
+        hasher.finish()
+    }
+
+    let k1 = serde_json::from_str::<Number>("0.0").unwrap();
+    let k2 = serde_json::from_str::<Number>("-0.0").unwrap();
+    assert!(k1 != k2 || hash(k1) == hash(k2));
+}