Allow for empty return expressions

Upsilon/Parser/Lexer.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Text;
@@ -198,6 +199,11 @@ namespace Upsilon.Parser
             {
                 return new IdentifierToken(str, start);
             }
+
+            if (kind == SyntaxKind.ReturnKeyword)
+            {
+                return new ReturnSyntaxToken(start, Next == Environment.NewLine[0]);
+            }
             return new SyntaxToken(kind, start, str, null);
         }
 
@@ -212,7 +218,7 @@ namespace Upsilon.Parser
                 if (Current != ' ')
                     stringBuilder.Append(Current);
             }
-            while (Next != '\n' && Next != '\0')
+            while (Next != Environment.NewLine[0] && Next != '\0')
             {
                 stringBuilder?.Append(Next);
                 _position++;

Upsilon/Parser/Parser.cs

@@ -282,9 +282,9 @@ namespace Upsilon.Parser
 
         private StatementSyntax ParseReturnStatement()
         {
-            var returnToken = MatchToken(SyntaxKind.ReturnKeyword);
+            var returnToken = (ReturnSyntaxToken)MatchToken(SyntaxKind.ReturnKeyword);
             ExpressionSyntax expression = null;
-            if (Current.Kind != SyntaxKind.EndKeyword)
+            if (Current.Kind != SyntaxKind.EndKeyword && !returnToken.FollowedByLineBreak)
                 expression = ParseExpression();
             return new ReturnStatementSyntax(returnToken, expression);
         }

Upsilon/Parser/SyntaxToken.cs

@@ -28,4 +28,15 @@ namespace Upsilon.Parser
             yield break;
         }
     }
+
+    public class ReturnSyntaxToken : SyntaxToken
+    {
+        public bool FollowedByLineBreak { get; }
+
+        public ReturnSyntaxToken(int position, bool followedByLineBreak)
+            : base(SyntaxKind.ReturnKeyword, position, "return", null)
+        {
+            FollowedByLineBreak = followedByLineBreak;
+        }
+    }
 }