Do not add a plus sign to a negative zero float result

tests/test_items.py

@@ -1451,6 +1451,19 @@ def test_no_extra_minus_sign() -> None:
     assert doc.as_string() == "a = -1.5"
 
 
+def test_no_extra_plus_sign_on_negative_zero() -> None:
+    doc = parse("a = -1.5")
+    doc["a"] *= 0
+    assert doc.as_string() == "a = -0.0"
+    doc["a"] *= -1
+    assert doc.as_string() == "a = +0.0"
+
+    # Integer true division also goes through Float._new.
+    doc = parse("a = +0")
+    doc["a"] /= -1
+    assert doc.as_string() == "a = -0.0"
+
+
 def test_serialize_table_with_dotted_key() -> None:
     child = api.table()
     child.add(api.key(("b", "c")), 1)

tomlkit/items.py

@@ -876,7 +876,8 @@ def as_string(self) -> str:
     def _new(self, result: float) -> Float:
         raw = str(result)
 
-        if self._sign and result >= 0:
+        # -0.0 >= 0 is True, but str(-0.0) already carries its sign.
+        if self._sign and result >= 0 and not raw.startswith("-"):
             raw = f"+{raw}"
 
         return Float(result, self._trivia, raw)