Return instead of yield from Concurrent::Promise

CHANGELOG.md

@@ -4,6 +4,7 @@ Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprocket
 
 ## Master
 
+- Fix `Manifest#find` yielding from a Promise causing issue on Ruby 3.1.0-dev. [#720](https://github.com/rails/sprockets/pull/720)
 - Better detect the ERB version to avoid deprecation warnings. [#719](https://github.com/rails/sprockets/pull/719)
 - Allow assets already fingerprinted to be served through `Sprockets::Server`
 - Do not fingerprint files that already contain a valid digest in their name

lib/sprockets/manifest.rb

@@ -112,7 +112,7 @@ def files
     # Public: Find all assets matching pattern set in environment.
     #
     # Returns Enumerator of Assets.
-    def find(*args)
+    def find(*args, &block)
       unless environment
         raise Error, "manifest requires environment for compilation"
       end
@@ -122,12 +122,13 @@ def find(*args)
       environment = self.environment.cached
       promises = args.flatten.map do |path|
         Concurrent::Promise.execute(executor: executor) do
-          environment.find_all_linked_assets(path) do |asset|
-            yield asset
-          end
+          environment.find_all_linked_assets(path).to_a
         end
       end
-      promises.each(&:wait!)
+
+      promises.each do |promise|
+        promise.value!.each(&block)
+      end
 
       nil
     end