fix #5729: raise TypeError when @app.template_filter is used without parenthesis

src/flask/sansio/app.py

@@ -672,10 +672,19 @@ def template_filter(
           def reverse(s):
               return s[::-1]
 
+        You must include parentheses when using this decorator,
+        even if no arguments are provided.
+
         :param name: the optional name of the filter, otherwise the
                      function name will be used.
         """
 
+        if callable(name):
+            raise TypeError(
+                "Did you mean to use @app.template_filter()? "
+                "Missing parentheses () in decorator"
+            )
+
         def decorator(f: T_template_filter) -> T_template_filter:
             self.add_template_filter(f, name=name)
             return f

tests/test_templating.py

@@ -130,6 +130,14 @@ def my_reverse(s):
     assert app.jinja_env.filters["my_reverse"]("abcd") == "dcba"
 
 
+def test_template_filter_requires_parentheses(app):
+    with pytest.raises(TypeError):
+
+        @app.template_filter
+        def my_reverse(s):
+            return s[::-1]
+
+
 def test_add_template_filter(app):
     def my_reverse(s):
         return s[::-1]