Fix OneOf parsers merge loosing SkipWhiteSpace

Fixes #190

src/Parlot/Fluent/OneOf.cs

@@ -26,6 +26,7 @@ public sealed class OneOf<T> : Parser<T>, ISeekable /*, ICompilable*/
     public OneOf(Parser<T>[] parsers)
     {
         Parsers = parsers ?? throw new ArgumentNullException(nameof(parsers));
+        OriginalParsers = parsers;
 
         // We can't build a lookup table if there is only one parser
         if (Parsers.Count <= 1)
@@ -151,6 +152,11 @@ public OneOf(Parser<T>[] parsers)
 
     public bool SkipWhitespace { get; }
 
+    /// <summary>
+    /// Gets the parsers before they get SkipWhitespace removed.
+    /// </summary>
+    public IReadOnlyList<Parser<T>> OriginalParsers { get; }
+
     public IReadOnlyList<Parser<T>> Parsers { get; }
 
     public override bool Parse(ParseContext context, ref ParseResult<T> result)

src/Parlot/Fluent/Parsers.OneOf.cs

@@ -1,4 +1,4 @@
-namespace Parlot.Fluent;
+namespace Parlot.Fluent;
 
 // We don't care about the performance of these helpers since they are called only once 
 // during the parser tree creation
@@ -16,7 +16,7 @@ public static Parser<T> Or<T>(this Parser<T> parser, Parser<T> or)
         if (parser is OneOf<T> oneOf)
         {
             // Return a single OneOf instance with this new one
-            return new OneOf<T>([.. oneOf.Parsers, or]);
+            return new OneOf<T>([.. oneOf.OriginalParsers, or]);
         }
         else
         {

test/Parlot.Tests/RewriteTests.cs

@@ -1,5 +1,6 @@
 using Parlot.Fluent;
 using Parlot.Rewriting;
+using System;
 using System.Collections.Generic;
 using System.Reflection;
 using Xunit;
@@ -280,4 +281,19 @@ public void OneOfShouldFindNonSeekableWithSpace(bool compiled, bool skipWhiteSpa
         Assert.Equal("c", p.Parse(" b"));
         Assert.Equal("c", p.Parse(" c"));
     }
+
+    [Theory]
+    [InlineData(" a ")]
+    [InlineData(" b ")]
+    [InlineData(" c ")]
+    public void MergingOneOfShouldRestoreSkipWhiteSpace(string input)
+    {
+        // The first two parsers are evaluated in a OneOf with SkipWhiteSpace stripped off. The second creation should
+        // restore these SkipWhiteSpace parsers for the lookup table to be re-created correctly.
+
+        var p = Terms.Char('a').Or(Terms.Char('b')).Or(Terms.Char('c'));
+
+        var result1 = p.TryParse(input, out _);
+        Assert.True(result1);
+    }
 }