Uphold guarantee that the resulting NaiveDateTime is always valid

Author: pitdicker

src/naive/date.rs

@@ -605,7 +605,9 @@ impl NaiveDate {
     /// ```
     #[must_use]
     pub fn checked_add_months(self, months: Months) -> Option<Self> {
-        if months.0 == 0 {
+        // To be useable by `DateTime` this method should guarantee the result is valid, even if
+        // the input `self` is out of range. So only short-circuit if in range.
+        if months.0 == 0 && self >= Self::MIN && self <= Self::MAX {
             return Some(self);
         }
 
@@ -636,7 +638,9 @@ impl NaiveDate {
     /// ```
     #[must_use]
     pub fn checked_sub_months(self, months: Months) -> Option<Self> {
-        if months.0 == 0 {
+        // To be useable by `DateTime` this method should guarantee the result is valid, even if
+        // the input `self` is out of range. So only short-circuit if in range.
+        if months.0 == 0 && self >= Self::MIN && self <= Self::MAX {
             return Some(self);
         }