Plugin.swift

protocol Commands { }

extension Command: Commands { }

@_functionBuilder
struct CommandsBuilder: Commands {
    static func buildBlock(_ commands: Command...) -> CommandList {
        return CommandList(commands: { commands })
    }
    
    static func buildExpression(_ commands: Command...) -> CommandList {
        return CommandList(commands: { commands })
    }
}

@_functionBuilder
struct CommandListBuilder: Commands {
    static func buildBlock(_ commands: Command...) -> [Command] {
        return commands
    }
}

struct CommandList: Commands {
    var commands: [Command]
    
    init(@CommandListBuilder commands: () -> [Command]) {
        self.commands = commands()
    }
}


protocol Plugin {
    
    associatedtype C: Commands
    
//    var otherCommands: () -> Commands { get }
    
    var commands: C { get }
}

struct PluginOne: Plugin {
    
    @CommandsBuilder
    var commands: some Commands {
        // Error here
        Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in
            
        }
        Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in
            
        }
    }
}

struct PluginTwo: Plugin {
    
    var commands: some Commands {
        // Works
        Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in
            
        }
    }
}

struct PluginThree: Plugin {
    
    var commands: some Commands {
        // Works
        CommandList {
            Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in
                
            }
            Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in
                
            }
        }
    }
}

Plugin_v2.swift

protocol Commands { }

extension Command: Commands { }

@_functionBuilder
struct CommandsBuilder: Commands {
    static func buildBlock(_ commands: Command...) -> CommandList {
        return CommandList(commands: { commands })
    }
    
    static func buildExpression(_ commands: Command...) -> CommandList {
        return CommandList(commands: { commands })
    }
}

@_functionBuilder
struct CommandListBuilder: Commands {
    static func buildBlock(_ commands: Command...) -> [Command] {
        return commands
    }
}

struct CommandList: Commands {
    var commands: [Command]
    
    init(@CommandListBuilder commands: () -> [Command]) {
        self.commands = commands()
    }
}


protocol Plugin {
    
    associatedtype C: Commands
    
    var commands: C { get }
}

@available(OSX 10.15.0, *)
struct PluginOne: Plugin {

    @CommandsBuilder
    var commands: some Commands {
        // Error here. Cannot convert return expression of type 'CommandList' to return type 'some Commands'
        Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in

        }
        Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in

        }
    }
}

@available(OSX 10.15.0, *)
struct PluginTwo: Plugin {

    var commands: some Commands {
        // Works
        Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in

        }
    }
}

@available(OSX 10.15.0, *)
struct PluginThree: Plugin {

    var commands: some Commands {
        // Works
        CommandList {
            Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in

            }
            Command(trigger: "abc", arguments: [], aliases: [], group: nil, permissionChecks: [], userInfo: [:]) { (_, _, _) in

            }
        }
    }
}