Field names should be rejected if they duplicate another field name

[chwarr]

The following .bond file should fail gbc code generation, as it contains two fields with the same name.

namespace duplicate_field_names;

struct ShouldBeRejected
{
    0: bool DuplicateFieldName;
    1: bool DuplicateFieldName;
}

However, Bond Compiler 0.4.0.1 accepts this .bond file and generates C++ and C# code that does not compile.

Here's an excerpt of the generated C# code.

namespace duplicate_field_names
{
    using System.Collections.Generic;

    [global::Bond.Schema]
    [System.CodeDom.Compiler.GeneratedCode("gbc", "0.4.0.1")]
    public partial class ShouldBeRejected
    {
        [global::Bond.Id(0)]
        public bool DuplicateFieldName { get; set; }

        [global::Bond.Id(1)]
        public bool DuplicateFieldName { get; set; }

        public ShouldBeRejected()
            : this("duplicate_field_names.ShouldBeRejected", "ShouldBeRejected")
        {}

        protected ShouldBeRejected(string fullName, string name)
        {

        }
    }
} // duplicate_field_names

[lalo]

For this one I was thinking of this simple change in Parser.h line 235:

    unique p = do
        fields' <- p
        case findDuplicates fields' fieldOrdinal ++ findDuplicates fields' fieldName of
            [] -> return fields'
            Field {..}:_ -> fail $ "Duplicate definition of the field with ordinal " ++ show fieldOrdinal ++
                " and field name " ++ show fieldName
      where
        findDuplicates xs ft = deleteFirstsBy (ordinal ft) xs (nubBy (ordinal ft) xs)
        ordinal ft = (==) `on` ft

The bad thing is that the error message will display both values as possible duplications when only one is duplicated. If we want a more specific error message for each case I can refactor it in some other way that allows this.

[chwarr]

If we change the message to "Duplicate definition of the field with either ordinal 0 or field name foo", the text will cover both cases.