Misleading error message when using [<CustomEquality>] or [<CustomComparison>] with a non-structural type

[kbattocchi]

If you mistakenly add the [<CustomEquality>] or [CustomComparison>] (or both) attributes to a non-structural type then you get a very misleading error message:

[<CustomEquality>]
type T() =
    override __.Equals(_) = true

error FS0377: This type uses an invalid mix of the attributes 'NoEquality', 'ReferenceEquality', 'StructuralEquality', 'NoComparison' and 'StructuralComparison' attributes

This causes confusion (see e.g. this StackOverflow question).

[nojaf]

@vzarytovskii what could be done to improve this error message?

[snuup]

@nojaf Suggestions: 1. Explain valid combinations. 2. Add documentation about this error message, as far as I see there is none yet.

[nojaf]

@nojaf Explain what combination is valid!

Jeesh, pick another writing style to address me there pal.

[smoothdeveloper]

AFAIU, those attributes shouldn't be a valid thing when tucked on ad-hoc structs/classes, so it is good it is an error already.

For this particular case, the error message could be:

The attributes 'NoEquality', 'ReferenceEquality', 'StructuralEquality', 'NoComparison' and 'StructuralComparison' aren't meant to be used on adhoc classes or structures, nor enums, but only on discriminated unions or record types.

Only drawback of this message is, if we come up with more of such types where the attributes are invalid/valid, then the message will need tweak.

The current error message bothers me a bit with "attributes" being repeated.

In meantime, "Jeesh" isn't offended of being addressed in any style 🙂 and users are sometimes exclamatory when addressing the compiler as an entity or simply greeting others!