💥 Redo all higher order template functions

This does what we did for a few functions in the last few commits for
every function. We now use either the `std::invocable` concept or our
own `invocable_returning` concept wherever possible to make sure we pass
function types to these template functions, since constraint errors are
a lot more readable than template deduction errors. And instead of
having to specify the return type as a template argument, we now just
use `std::invoke_result_t<F>` instead. The VST3 message handling
functions are still using the good old `typename F` since those are
overloaded polymorphic functions. This was also a good moment to modify
`AdHocSocketHandler::send()` to allow functions returning void (this got
rid of an old fixme where we had to return some dummy value from a
function instead of just not returning anything).

src/plugin/bridges/common.h

@@ -51,14 +51,13 @@ class PluginBridge {
      *   Using a lambda here feels wrong, but I can't think of a better
      *   solution right now.
      *
-     * @tparam F A `TSockets(boost::asio::io_context&, const PluginInfo&)`
-     *   function to create the `TSockets` instance.
-     *
      * @throw std::runtime_error Thrown when the Wine plugin host could not be
      *   found, or if it could not locate and load a VST3 module.
      */
-    template <typename F>
-    PluginBridge(PluginType plugin_type, F create_socket_instance)
+    template <invocable_returning<TSockets,
+                                  boost::asio::io_context&,
+                                  const PluginInfo&> F>
+    PluginBridge(PluginType plugin_type, F&& create_socket_instance)
         // This is still correct for VST3 plugins because we can configure an
         // entire directory (the module's bundle) at once
         : config(load_config_for(get_this_file_location())),

src/plugin/bridges/vst3-impls/plug-view-proxy.h

@@ -136,12 +136,11 @@ class Vst3PlugViewProxyImpl : public Vst3PlugViewProxy {
      * right now.
      *
      * @see Vst3HostBridge::send_mutually_recursive_message
-     *
-     * TODO: As mentioned elsewhere, refactor these functions to use
-     *       `std::invoke_result_t`
      */
-    template <typename T, typename F>
-    T run_gui_task(F fn) {
+    template <std::invocable F>
+    std::invoke_result_t<F> run_gui_task(F&& fn) {
+        using Result = std::invoke_result_t<F>;
+
         // If `Vst3Bridge::send_mutually_recursive_message()` is currently being
         // called (because the host is calling one of `IPlugView`'s methods from
         // its UGI thread), then we'll call `fn` from that same thread.
@@ -154,8 +153,8 @@ class Vst3PlugViewProxyImpl : public Vst3PlugViewProxy {
         }
 
         if (run_loop_tasks) {
-            std::packaged_task<T()> do_call(std::move(fn));
-            std::future<T> do_call_response = do_call.get_future();
+            std::packaged_task<Result()> do_call(std::forward<F>(fn));
+            std::future<Result> do_call_response = do_call.get_future();
 
             run_loop_tasks->schedule(std::move(do_call));
 

src/plugin/bridges/vst3.cpp

@@ -219,7 +219,7 @@ Vst3PluginBridge::Vst3PluginBridge()
                     // loop or else it will likely segfault at some point
                     return plugin_proxies.at(request.owner_instance_id)
                         .get()
-                        .last_created_plug_view->run_gui_task<tresult>([&]() {
+                        .last_created_plug_view->run_gui_task([&]() -> tresult {
                             return plugin_proxies.at(request.owner_instance_id)
                                 .get()
                                 .context_menus.at(request.context_menu_id)
@@ -285,7 +285,7 @@ Vst3PluginBridge::Vst3PluginBridge()
 
                     // REAPER requires this to be run from its provided event
                     // loop or else it will likely segfault at some point
-                    return plug_view->run_gui_task<tresult>([&]() {
+                    return plug_view->run_gui_task([&]() -> tresult {
                         return plug_view->plug_frame->resizeView(
                             plug_view, &request.new_size);
                     });

src/plugin/bridges/vst3.h

@@ -165,7 +165,7 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
      *
      * @see Vst3PlugViewProxyImpl::run_gui_task
      */
-    template <typename F>
+    template <std::invocable F>
     std::optional<std::invoke_result_t<F>> maybe_run_on_mutual_recursion_thread(
         F&& fn) {
         return mutual_recursion.maybe_handle(std::forward<F>(fn));