fix: get back _blueprint_order, important for template searching

CHANGES.rst

@@ -8,6 +8,10 @@ Unreleased
 -   Fix type annotation for ``teardown_request``. :issue:`4093`
 -   Fix type annotation for ``before_request`` and ``before_app_request``
     decorators. :issue:`4104`
+-   Re-add the ``Flask._blueprint_order`` list. Take blueprint registration
+    order into consideration when searching for templates in
+    ``DispatchingJinjaLoader._iter_loaders()``,
+    ``DispatchingJinjaLoader.list_templates()``.
 
 
 Version 2.0.1

src/flask/app.py

@@ -463,6 +463,7 @@ class Flask(Scaffold):
         #:
         #: .. versionadded:: 0.7
         self.blueprints: t.Dict[str, "Blueprint"] = {}
+        self._blueprint_order: t.List[Blueprint] = []
 
         #: a place where extensions can store application specific state.  For
         #: example this is where an extension could store database engines and
@@ -1035,7 +1036,7 @@ class Flask(Scaffold):
 
         .. versionadded:: 0.11
         """
-        return self.blueprints.values()
+        return iter(self._blueprint_order)
 
     @setupmethod
     def add_url_rule(

src/flask/blueprints.py

@@ -319,6 +319,8 @@ class Blueprint(Scaffold):
                 )
 
         app.blueprints[name] = self
+        if self not in app._blueprint_order:
+            app._blueprint_order.append(self)
         self._got_registered_once = True
         state = self.make_setup_state(app, options, first_registration)