Fix Windows build

include/audacity/ModuleInterface.h

@@ -116,7 +116,8 @@ public:
    // even if some plugins are also discovered successfully.
    // Return value is the number of plugins found.
    using RegistrationCallback =
-      std::function< void(ModuleInterface *, EffectIdentInterface *) >;
+      std::function<
+         const PluginID &(ModuleInterface *, EffectIdentInterface *) >;
    virtual unsigned DiscoverPluginsAtPath(
       const wxString & path, wxString &errMsg,
       const RegistrationCallback &callback =

src/PluginManager.cpp

@@ -1761,8 +1761,8 @@ bool PluginManager::DropFile(const wxString &fileName)
       if (!ff.empty() && module->PathsAreFiles()) {
          wxString errMsg;
          // Do dry-run test of the file format
-         unsigned nPlugIns = module->DiscoverPluginsAtPath(fileName, errMsg,
+         unsigned nPlugIns =
-            [](ModuleInterface *, EffectIdentInterface *){});
+            module->DiscoverPluginsAtPath(fileName, errMsg, {});
          if (nPlugIns) {
             // File contents are good for this module, so check no others.
             // All branches of this block return true, even in case of
@@ -1813,10 +1813,12 @@ bool PluginManager::DropFile(const wxString &fileName)
                [&](ModuleInterface *provider, EffectIdentInterface *ident){
                   // Register as by default, but also collecting the PluginIDs
                   // and names
-                  ids.push_back(
+                  const auto &id =
                      PluginManagerInterface::DefaultRegistrationCallback(
-                        provider, ident) );
+                        provider, ident);
+                  ids.push_back(id);
                   names.push_back( wxGetTranslation( ident->GetName() ) );
+                  return id;
                });
             if ( ! nPlugIns ) {
                // Unlikely after the dry run succeeded

src/effects/LoadEffects.cpp

@@ -319,7 +319,8 @@ unsigned BuiltinEffectsModule::DiscoverPluginsAtPath(
    auto effect = Instantiate(path);
    if (effect)
    {
-      callback(this, effect.get());
+      if (callback)
+         callback(this, effect.get());
       return 1;
    }
 

src/effects/VST/VSTEffect.cpp

@@ -630,7 +630,8 @@ unsigned VSTEffectsModule::DiscoverPluginsAtPath(
                if (!skip && cont)
                {
                   valid = true;
-                  callback( this, &proc );
+                  if (callback)
+                     callback( this, &proc );
                   ++nFound;
                }
             }

src/effects/audiounits/AudioUnitEffect.cpp

@@ -193,7 +193,9 @@ unsigned AudioUnitEffectsModule::DiscoverPluginsAtPath(
       return 0;
    }
 
-   callback(this, &effect);
+   if(callback)
+      callback(this, &effect);
+
    return 1;
 }
 

src/effects/ladspa/LadspaEffect.cpp

@@ -250,7 +250,8 @@ unsigned LadspaEffectsModule::DiscoverPluginsAtPath(
             LadspaEffect effect(path, index);
             if (effect.SetHost(NULL)) {
                ++nLoaded;
-               callback( this, &effect );
+               if (callback)
+                  callback( this, &effect );
             }
             else
                errMsg = _("Could not load the library");

src/effects/lv2/LoadLV2.cpp

@@ -253,7 +253,8 @@ unsigned LV2EffectsModule::DiscoverPluginsAtPath(
       LV2Effect effect(plug);
       if (effect.SetHost(NULL))
       {
-         callback( this, &effect );
+         if (callback)
+            callback( this, &effect );
          return 1;
       }
    }

src/effects/nyquist/LoadNyquist.cpp

@@ -223,7 +223,8 @@ unsigned NyquistEffectsModule::DiscoverPluginsAtPath(
    NyquistEffect effect(path);
    if (effect.IsOk())
    {
-      callback(this, &effect);
+      if (callback)
+         callback(this, &effect);
       return 1;
    }
 

src/effects/vamp/LoadVamp.cpp

@@ -211,7 +211,8 @@ unsigned VampEffectsModule::DiscoverPluginsAtPath(
    if (vp)
    {
       VampEffect effect(std::move(vp), path, output, hasParameters);
-      callback( this, &effect );
+      if (callback)
+         callback( this, &effect );
 
       return 1;
    }