Nested blueprints

This allows blueprints to be nested within blueprints via a new
Blueprint.register_blueprint method. This should provide a use case
that has been desired for the past ~10 years.

This works by setting the endpoint name to be the blueprint names,
from parent to child delimeted by "." and then iterating over the
blueprint names in reverse order in the app (from most specific to
most general). This means that the expectation of nesting a blueprint
within a nested blueprint is met.

src/flask/blueprints.py

@@ -45,6 +45,8 @@ class BlueprintSetupState:
         #: blueprint.
         self.url_prefix = url_prefix
 
+        self.name_prefix = self.options.get("name_prefix", "")
+
         #: A dictionary with URL defaults that is added to each and every
         #: URL that was defined with the blueprint.
         self.url_defaults = dict(self.blueprint.url_values_defaults)
@@ -68,7 +70,7 @@ class BlueprintSetupState:
             defaults = dict(defaults, **options.pop("defaults"))
         self.app.add_url_rule(
             rule,
-            f"{self.blueprint.name}.{endpoint}",
+            f"{self.name_prefix}{self.blueprint.name}.{endpoint}",
             view_func,
             defaults=defaults,
             **options,
@@ -168,6 +170,7 @@ class Blueprint(Scaffold):
 
         self.url_values_defaults = url_defaults
         self.cli_group = cli_group
+        self._blueprints = []
 
     def _is_setup_finished(self):
         return self.warn_on_modifications and self._got_registered_once
@@ -210,7 +213,16 @@ class Blueprint(Scaffold):
         """
         return BlueprintSetupState(self, app, options, first_registration)
 
-    def register(self, app, options, first_registration=False):
+    def register_blueprint(self, blueprint, **options):
+        """Register a :class:`~flask.Blueprint` on this blueprint. Keyword
+        arguments passed to this method will override the defaults set
+        on the blueprint.
+
+        .. versionadded:: 2.0
+        """
+        self._blueprints.append((blueprint, options))
+
+    def register(self, app, options):
         """Called by :meth:`Flask.register_blueprint` to register all
         views and callbacks registered on the blueprint with the
         application. Creates a :class:`.BlueprintSetupState` and calls
@@ -223,6 +235,20 @@ class Blueprint(Scaffold):
         :param first_registration: Whether this is the first time this
             blueprint has been registered on the application.
         """
+        first_registration = False
+
+        if self.name in app.blueprints:
+            assert app.blueprints[self.name] is self, (
+                "A name collision occurred between blueprints"
+                f" {self!r} and {app.blueprints[self.name]!r}."
+                f" Both share the same name {self.name!r}."
+                f" Blueprints that are created on the fly need unique"
+                f" names."
+            )
+        else:
+            app.blueprints[self.name] = self
+            first_registration = True
+
         self._got_registered_once = True
         state = self.make_setup_state(app, options, first_registration)
 
@@ -278,19 +304,28 @@ class Blueprint(Scaffold):
         for deferred in self.deferred_functions:
             deferred(state)
 
-        if not self.cli.commands:
-            return
-
         cli_resolved_group = options.get("cli_group", self.cli_group)
 
-        if cli_resolved_group is None:
-            app.cli.commands.update(self.cli.commands)
-        elif cli_resolved_group is _sentinel:
-            self.cli.name = self.name
-            app.cli.add_command(self.cli)
-        else:
-            self.cli.name = cli_resolved_group
-            app.cli.add_command(self.cli)
+        if self.cli.commands:
+            if cli_resolved_group is None:
+                app.cli.commands.update(self.cli.commands)
+            elif cli_resolved_group is _sentinel:
+                self.cli.name = self.name
+                app.cli.add_command(self.cli)
+            else:
+                self.cli.name = cli_resolved_group
+                app.cli.add_command(self.cli)
+
+        for blueprint, bp_options in self._blueprints:
+            url_prefix = options.get("url_prefix", "")
+            if "url_prefix" in bp_options:
+                url_prefix = (
+                    url_prefix.rstrip("/") + "/" + bp_options["url_prefix"].lstrip("/")
+                )
+
+            bp_options["url_prefix"] = url_prefix
+            bp_options["name_prefix"] = options.get("name_prefix", "") + self.name + "."
+            blueprint.register(app, bp_options)
 
     def add_url_rule(self, rule, endpoint=None, view_func=None, **options):
         """Like :meth:`Flask.add_url_rule` but for a blueprint.  The endpoint for