Address review comments from pallets#1560.

- Defer adding the static route until right before the first request.
- No longer set url_map.host_matching to True implicitly if static_host is
  provided. Instead raise an error at the time the static route is added if
  static_host is truthy and host_matching is falsy.

flask/app.py

@@ -131,8 +131,8 @@ class Flask(_PackageBoundObject):
                           at `static_url_path`.  Defaults to the ``'static'``
                           folder in the root path of the application.
     :param static_host: the host to use when adding the static route.
-                        Defaults to None. Setting this implicitly causes
-                        ``app.url_map.host_matching`` to be set to True.
+                        Defaults to None. Only use if setting
+                        ``app.url_map.host_matching`` to True.
     :param template_folder: the folder that contains the templates that should
                             be used by the application.  Defaults to
                             ``'templates'`` folder in the root path of the
@@ -534,19 +534,27 @@ def __init__(self, import_name, static_path=None, static_url_path=None,
         self._got_first_request = False
         self._before_request_lock = Lock()
 
-        # Register the static folder for the application if one is configured.
-        # Note we do this without checking if the configured folder exists.
+        # Create a callback that adds a route for static requests using the
+        # provided static_url_path, static_host, and static_folder,
+        # iff there is a configured static_folder.
+        # Note we do this without checking if static_folder exists.
         # For one, it might be created while the server is running (e.g. during
         # development). Also, Google App Engine stores static files somewhere
         # else when mapped with the .yml file.
-        if self.has_static_folder:
-            if static_host:  # Passing static_host implies host_matching = True.
-                # Must be set before adding the url rule or else it won't match.
-                self.url_map.host_matching = True
+        # This is deferred until right before the first request, because the
+        # user may set app.url_map.host_matching to True after the app has
+        # been initialized, and host_matching needs to be checked for at the
+        # time the route is added for requests to be routed correctly.
+        self._should_add_static_route = self.has_static_folder
+        def add_static_route():
+            assert not static_host or self.url_map.host_matching, (
+                'Expected app.url_map.host_matching to be True when static_host provided')
             self.add_url_rule(self.static_url_path + '/<path:filename>',
                               endpoint='static', host=static_host,
                               view_func=self.send_static_file)
 
+        self._add_static_route = add_static_route
+
         #: The click command line context for this application.  Commands
         #: registered here show up in the :command:`flask` command once the
         #: application has been discovered.  The default commands are
@@ -1989,6 +1997,10 @@ def wsgi_app(self, environ, start_response):
                                a list of headers and an optional
                                exception context to start the response
         """
+        with self._before_request_lock:
+            if not self._got_first_request and self._should_add_static_route:
+                self._add_static_route()
+                self._should_add_static_route = False
         ctx = self.request_context(environ)
         ctx.push()
         error = None

tests/test_basic.py

@@ -1173,7 +1173,7 @@ def test_static_url_path():
 
 def test_static_route_with_host_matching():
     app = flask.Flask(__name__, static_host='example.com')
-    assert app.url_map.host_matching, 'passing static_host implies host_matching = True'
+    app.url_map.host_matching = True
     c = app.test_client()
     assert c.get('http://example.com/static/index.html').status_code == 200
     with app.test_request_context():