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Reword or better explain unique names on Struct DUs #5236

@isaacabraham

Description

@isaacabraham

What

When creating a multi-case struct DU, if there is more than one field across all cases, each fields across all cases must have explicits, unique name e.g.

[<Struct>]
type Foo =
    | Hi of bool
    | Lo of int

leads to

error FS3204: If a union type has more than one case and is a struct, then all fields within the union type must be given unique names

Why

The error is misleading IMHO. Firstly, it's not clear to me under what circumstances this error appears e.g. this compiles fine:

[<Struct>]
type Foo =
| Hi of bool
| Lo of int * bool

Nonetheless, the main issue is that the error message makes me think that all named fields must be unique i.e. surely the compiler can generate unique names for the other ones?

Yet this works:

[<Struct>]
type Foo =
| Hi of bool
| Lo of bar:int

But this doesn't

[<Struct>]
type Foo =
| Hi of bool
| Lo of bar:int
| Med of int

In short, the compiler error message doesn't clearly explain when this error occurs, or how to resolve it.

How

The error message should clearly state how this error occurred (I would suggest something but I'm not entirely clear myself at the moment), and it should ideally give a hint as to how to fix the issue with the offending fields that have duplicated names.

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