Fix InvalidCastException in Then<U>() for non-IConvertible types

src/Parlot/Fluent/Parser.cs

@@ -25,9 +25,42 @@ public abstract partial class Parser<T>
     public Parser<U> Then<U>(U value) => new Then<T, U>(this, value);
 
     /// <summary>
-    /// Builds a parser that converts the previous result.
+    /// Builds a parser that discards the previous result and returns the default value of type U.
+    /// For types that implement IConvertible, attempts type conversion.
     /// </summary>
-    public Parser<U?> Then<U>() => new Then<T, U?>(this, x => (U?)Convert.ChangeType(x, typeof(U?), CultureInfo.CurrentCulture));
+    public Parser<U?> Then<U>()
+    {
+        // Check if U implements IConvertible at construction time for performance
+        var targetImplementsIConvertible = typeof(IConvertible).IsAssignableFrom(typeof(U));
+
+        if (targetImplementsIConvertible)
+        {
+            return new Then<T, U?>(this, x =>
+            {
+                // If both T and U are IConvertible, try to convert
+                if (x is IConvertible)
+                {
+                    try
+                    {
+                        return (U?)Convert.ChangeType(x, typeof(U), CultureInfo.CurrentCulture);
+                    }
+                    catch
+                    {
+                        // Fall back to default if conversion fails
+                        return default(U);
+                    }
+                }
+
+                // For non-convertible types, return default
+                return default(U);
+            });
+        }
+        else
+        {
+            // For types that don't implement IConvertible, just return default
+            return new Then<T, U?>(this, default(U));
+        }
+    }
 
     /// <summary>
     /// Builds a parser that converts the previous result when it succeeds or returns a default value if it fails.

test/Parlot.Tests/ThenIssueTests.cs

@@ -0,0 +1,40 @@
+using Parlot.Fluent;
+using Xunit;
+
+using static Parlot.Fluent.Parsers;
+
+namespace Parlot.Tests;
+
+#nullable enable
+
+public class ThenIssueTests
+{
+    [Theory]
+    [InlineData("""
+        // A Comment
+        var a = 'test'
+        """)]
+    [InlineData("""
+        var a = 'test'
+        """)]
+    public void ThenWithNonIConvertibleTypeShouldWork(string testString)
+    {
+        var commentParser = Terms.Text("//").And(AnyCharBefore(OneOf(Literals.Char('\n'), Literals.Char('\r'))));
+        var varKeyword = Terms.Text("var");
+        var identifier = Terms.Identifier();
+        var equal = Terms.Char('=');
+        var str = Terms.String();
+
+        var expressionParser = varKeyword.And(identifier).And(equal).And(str).Then<BaseExp?>(a => new AssignExpression(a.Item2.ToString()!, a.Item4.ToString()!));
+
+        var simpleParser = OneOf(commentParser.Then<BaseExp>(), expressionParser);
+
+        var success = simpleParser.TryParse(testString, out var result);
+
+        Assert.True(success);
+    }
+
+    public abstract record BaseExp;
+
+    public record AssignExpression(string Name, string Value) : BaseExp;
+}