Support must-understand response directive

src/werkzeug/datastructures/cache_control.py

@@ -179,6 +179,8 @@ class ResponseCacheControl(_CacheControl):
 
        `no_transform` is now a boolean instead of returning `None` whether
        the directive is present or not.
+
+       `must_understand` added.
     """
 
     public = cache_control_property("public", None, bool)
@@ -187,6 +189,7 @@ class ResponseCacheControl(_CacheControl):
     proxy_revalidate = cache_control_property("proxy-revalidate", None, bool)
     s_maxage = cache_control_property("s-maxage", None, int)
     immutable = cache_control_property("immutable", None, bool)
+    must_understand = cache_control_property("must-understand", None, bool)
 
 
 # circular dependencies

src/werkzeug/datastructures/cache_control.pyi

@@ -110,3 +110,9 @@ class ResponseCacheControl(_CacheControl):
     def immutable(self, value: bool | None) -> None: ...
     @immutable.deleter
     def immutable(self) -> None: ...
+    @property
+    def must_understand(self) -> bool: ...
+    @must_understand.setter
+    def must_understand(self, value: bool | None) -> None: ...
+    @must_understand.deleter
+    def must_understand(self) -> None: ...

tests/test_datastructures.py

@@ -967,6 +967,12 @@ def test_min_fresh(self):
         cc = ds.RequestCacheControl()
         assert cc.min_fresh is None
 
+    def test_must_understand(self):
+        cc = ds.ResponseCacheControl([("must-understand", None)])
+        assert cc.must_understand is True
+        cc = ds.ResponseCacheControl()
+        assert cc.must_understand is False
+
 
 class TestContentSecurityPolicy:
     def test_construct(self):