Copy trivia in _new so arithmetic results cannot mutate the original item

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Arithmetic and `replace()` results for `Integer`, `Float`, `String`, `DateTime`, `Date` and `Time` no longer share the original item's `Trivia`, so changing the result's comment or indentation no longer rewrites the item still stored in the document.
+
 ## [0.15.2] - 2026-10-16
 
 ### Changed

tests/test_items.py

@@ -701,6 +701,31 @@ def test_item_array_of_dicts_converted_to_aot() -> None:
     )
 
 
+@pytest.mark.parametrize(
+    ("content", "op"),
+    [
+        ("my_int = 1  # keep\n", lambda item: item + 1),
+        ("my_float = 1.5  # keep\n", lambda item: item + 1),
+        ('my_str = "a"  # keep\n', lambda item: item + "b"),
+        (
+            "my_dt = 1979-05-27T07:32:00Z  # keep\n",
+            lambda item: item + timedelta(days=1),
+        ),
+        ("my_date = 1979-05-27  # keep\n", lambda item: item + timedelta(days=1)),
+        ("my_time = 07:32:00  # keep\n", lambda item: item.replace(second=1)),
+    ],
+)
+def test_arithmetic_result_does_not_share_trivia(content: str, op: Callable) -> None:
+    doc = parse(content)
+    original = doc[next(iter(doc))]
+
+    result = op(original)
+    assert result.trivia is not original.trivia
+
+    result.trivia.comment = "# MUTATED"
+    assert doc.as_string() == content
+
+
 def test_add_float_to_int() -> None:
     content = "[table]\nmy_int = 2043"
     doc = parse(content)

tomlkit/items.py

@@ -671,7 +671,7 @@ def _new(self, result: int) -> Integer:
         if self._sign and result >= 0:
             raw = f"+{raw}"
 
-        return Integer(result, self._trivia, raw)
+        return Integer(result, self._trivia.copy(), raw)
 
     def _getstate(self, protocol: int = 3) -> tuple[int, Trivia, str]:
         return int(self), self._trivia, self._raw
@@ -896,7 +896,7 @@ def _new(self, result: float) -> Float:
         if self._sign and result >= 0 and not raw.startswith("-"):
             raw = f"+{raw}"
 
-        return Float(result, self._trivia, raw)
+        return Float(result, self._trivia.copy(), raw)
 
     def _getstate(self, protocol: int = 3) -> tuple[float, Trivia, str]:
         return float(self), self._trivia, self._raw
@@ -1202,7 +1202,7 @@ def _new(self, result: datetime) -> DateTime:
             result.second,
             result.microsecond,
             result.tzinfo,
-            self._trivia,
+            self._trivia.copy(),
             raw,
             fold=result.fold,
         )
@@ -1297,7 +1297,7 @@ def replace(self, *args: object, **kwargs: object) -> Date:
     def _new(self, result: date) -> Date:
         raw = result.isoformat()
 
-        return Date(result.year, result.month, result.day, self._trivia, raw)
+        return Date(result.year, result.month, result.day, self._trivia.copy(), raw)
 
     def _getstate(self, protocol: int = 3) -> tuple[int, int, int, Trivia, str]:
         return (self.year, self.month, self.day, self._trivia, self._raw)
@@ -1363,7 +1363,7 @@ def _new(self, result: time) -> Time:
             result.second,
             result.microsecond,
             result.tzinfo,
-            self._trivia,
+            self._trivia.copy(),
             raw,
             fold=result.fold,
         )
@@ -2291,7 +2291,7 @@ def __add__(self, other: str) -> String:
         return self._new(result, original)
 
     def _new(self, result: str, original: str) -> String:
-        return String(self._t, result, original, self._trivia)
+        return String(self._t, result, original, self._trivia.copy())
 
     def _getstate(self, protocol: int = 3) -> tuple[StringType, str, str, Trivia]:
         return self._t, str(self), self._original, self._trivia