Switch to IReadOnlyCollection<T> and fix compiler errors

src/Discord.Net.Commands/CommandMatch.cs

@@ -16,9 +16,9 @@ namespace Discord.Commands
             Alias = alias;
         }
 
-        public Task<PreconditionResult> CheckPreconditionsAsync(ICommandContext context, IServiceProvider services = null)
+        public Task<IResult> CheckPreconditionsAsync(ICommandContext context, IServiceProvider services = null)
             => Command.CheckPreconditionsAsync(context, services);
-        public Task<ParseResult> ParseAsync(ICommandContext context, SearchResult searchResult, PreconditionResult? preconditionResult = null)
+        public Task<ParseResult> ParseAsync(ICommandContext context, SearchResult searchResult, IResult preconditionResult = null)
             => Command.ParseAsync(context, Alias.Length, searchResult, preconditionResult);
         public Task<ExecuteResult> ExecuteAsync(ICommandContext context, IEnumerable<object> argList, IEnumerable<object> paramList, IServiceProvider services)
             => Command.ExecuteAsync(context, argList, paramList, services);

src/Discord.Net.Commands/Info/CommandInfo.cs

@@ -105,12 +105,12 @@ namespace Discord.Commands
             return PreconditionResult.FromSuccess();
         }
         
-        public async Task<ParseResult> ParseAsync(ICommandContext context, int startIndex, SearchResult searchResult, PreconditionResult? preconditionResult = null)
+        public async Task<ParseResult> ParseAsync(ICommandContext context, int startIndex, SearchResult searchResult, IResult preconditionResult = null)
         {
             if (!searchResult.IsSuccess)
                 return ParseResult.FromError(searchResult);
-            if (preconditionResult != null && !preconditionResult.Value.IsSuccess)
-                return ParseResult.FromError(preconditionResult.Value);
+            if (preconditionResult != null && !preconditionResult.IsSuccess)
+                return ParseResult.FromError(preconditionResult);
             
             string input = searchResult.Text.Substring(startIndex);
             return await CommandParser.ParseArgs(this, context, input, 0).ConfigureAwait(false);

src/Discord.Net.Commands/Results/PreconditionGroupResult.cs

@@ -13,18 +13,18 @@ namespace Discord.Commands
 
         public bool IsSuccess => !Error.HasValue;
 
-        public IEnumerable<PreconditionResult> Preconditions { get; }
+        public IReadOnlyCollection<PreconditionResult> Preconditions { get; }
 
-        private PreconditionGroupResult(CommandError? error, string errorReason, IEnumerable<PreconditionResult> preconditions)
+        private PreconditionGroupResult(CommandError? error, string errorReason, ICollection<PreconditionResult> preconditions)
         {
             Error = error;
             ErrorReason = errorReason;
-            Preconditions = preconditions ?? Enumerable.Empty<PreconditionResult>();
+            Preconditions = (preconditions ?? Enumerable.Empty<PreconditionResult>().ToList()).ToReadOnlyCollection();
         }
 
         public static PreconditionGroupResult FromSuccess()
             => new PreconditionGroupResult(null, null, null);
-        public static PreconditionGroupResult FromError(string reason, IEnumerable<PreconditionResult> preconditions)
+        public static PreconditionGroupResult FromError(string reason, ICollection<PreconditionResult> preconditions)
             => new PreconditionGroupResult(CommandError.UnmetPrecondition, reason, preconditions);
         public static PreconditionGroupResult FromError(IResult result) //needed?
             => new PreconditionGroupResult(result.Error, result.ErrorReason, null);