Improve decorator factory type signatures

These changes are required to preserve the type signatures of the
created decorators.

src/flask/app.py

@@ -1089,7 +1089,9 @@ class Flask(Scaffold):
             self.view_functions[endpoint] = view_func
 
     @setupmethod
-    def template_filter(self, name: t.Optional[str] = None) -> t.Callable:
+    def template_filter(
+        self, name: t.Optional[str] = None
+    ) -> t.Callable[[TemplateFilterCallable], TemplateFilterCallable]:
         """A decorator that is used to register custom template filter.
         You can specify a name for the filter, otherwise the function
         name will be used. Example::
@@ -1121,7 +1123,9 @@ class Flask(Scaffold):
         self.jinja_env.filters[name or f.__name__] = f
 
     @setupmethod
-    def template_test(self, name: t.Optional[str] = None) -> t.Callable:
+    def template_test(
+        self, name: t.Optional[str] = None
+    ) -> t.Callable[[TemplateTestCallable], TemplateTestCallable]:
         """A decorator that is used to register custom template test.
         You can specify a name for the test, otherwise the function
         name will be used. Example::
@@ -1162,7 +1166,9 @@ class Flask(Scaffold):
         self.jinja_env.tests[name or f.__name__] = f
 
     @setupmethod
-    def template_global(self, name: t.Optional[str] = None) -> t.Callable:
+    def template_global(
+        self, name: t.Optional[str] = None
+    ) -> t.Callable[[TemplateGlobalCallable], TemplateGlobalCallable]:
         """A decorator that is used to register a custom template global function.
         You can specify a name for the global function, otherwise the function
         name will be used. Example::

src/flask/blueprints.py

@@ -387,7 +387,9 @@ class Blueprint(Scaffold):
             )
         )
 
-    def app_template_filter(self, name: t.Optional[str] = None) -> t.Callable:
+    def app_template_filter(
+        self, name: t.Optional[str] = None
+    ) -> t.Callable[[TemplateFilterCallable], TemplateFilterCallable]:
         """Register a custom template filter, available application wide.  Like
         :meth:`Flask.template_filter` but for a blueprint.
 
@@ -417,7 +419,9 @@ class Blueprint(Scaffold):
 
         self.record_once(register_template)
 
-    def app_template_test(self, name: t.Optional[str] = None) -> t.Callable:
+    def app_template_test(
+        self, name: t.Optional[str] = None
+    ) -> t.Callable[[TemplateTestCallable], TemplateTestCallable]:
         """Register a custom template test, available application wide.  Like
         :meth:`Flask.template_test` but for a blueprint.
 
@@ -451,7 +455,9 @@ class Blueprint(Scaffold):
 
         self.record_once(register_template)
 
-    def app_template_global(self, name: t.Optional[str] = None) -> t.Callable:
+    def app_template_global(
+        self, name: t.Optional[str] = None
+    ) -> t.Callable[[TemplateGlobalCallable], TemplateGlobalCallable]:
         """Register a custom template global, available application wide.  Like
         :meth:`Flask.template_global` but for a blueprint.
 

src/flask/scaffold.py

@@ -644,7 +644,7 @@ class Scaffold:
     @setupmethod
     def errorhandler(
         self, code_or_exception: t.Union[t.Type[Exception], int]
-    ) -> t.Callable:
+    ) -> t.Callable[[ErrorHandlerCallable], ErrorHandlerCallable]:
         """Register a function to handle errors by code or exception class.
 
         A decorator that is used to register a function given an