parsers: report the name when a named Choice fails

src/Ramstack.Parsing/Parser.Choice.cs

@@ -110,6 +110,7 @@ public override bool TryParse(ref ParseContext context, [NotNullWhen(true)] out
             }
 
             context.RestoreDiagnosticState(state);
+            context.ReportExpected(Name);
             value = default;
             return false;
         }

tests/Ramstack.Parsing.Tests/ParsersTests.Choice.cs

@@ -47,4 +47,26 @@ public void ChoiceTest_2()
             parsers.Map(m => (m.Index, m.Length)).Parse("Sunset").Value,
             Is.EqualTo((0, 3)));
     }
+
+    [Test]
+    public void Choice_NamedParser_ReportsName()
+    {
+        // The unnamed alternative makes this a ChoiceParser rather than
+        // DeferredDiagnosticChoiceParser.
+        var parser = Choice(L('a'), L('b').Between(L('('), L(')'))).As("value");
+
+        Assert.That(
+            parser.Parse("?").ErrorMessage,
+            Is.EqualTo("(1:1) Expected value"));
+    }
+
+    [Test]
+    public void Choice_UnnamedParser_ReportsAlternatives()
+    {
+        var parser = Choice(L('a'), L('b').Between(L('('), L(')')));
+
+        Assert.That(
+            parser.Parse("?").ErrorMessage,
+            Is.EqualTo("(1:1) Expected 'a' or '('"));
+    }
 }