FSI generates internal error: Index not found with Struct DU

[cartermp]

1. Open FSI via command line or F# Interactive
2. Enter the following:

[<Struct>]
type Shape =
    | Circle of radius: float
    | Square of side: float
    | Triangle of height: float * width: float

Expected: FSI executes and the type is registered.
Actual: error FS0193: internal error: Index not found. (Exception from HResult: 0x80131124

[kbattocchi]

Are multi-case struct DUs supposed to work? I thought only single-case DUs could be represented as structs. (That is, I thought the expected behavior would be a compile-time error, not runtime success)

[cartermp]

@kbattocchi Multicase DUs are now supported in F# 4.1, although there is currently a bug with them: #1678

[kbattocchi]

I see; it looks like the RFC states

For multi-case struct unions, each case must have a unique name.

but presumably this was supposed to say unique type instead? In which case this would again be expected to fail at compile-time.

(As an aside, I had trouble finding the RFC because several of the links to it are dead since it was moved from /RFCs to /FSharp-4.1)

[cartermp]

@dsyme could comment further, but the intention to my understanding is that they simply require unique names. They shouldn't also require unique types.

[KevinRansom]

@kbattocchi
Well .. it works fine if it's not a struct so it seems like it should work for struct DUs too

Works in FSI

type StructShape =
    | Circle of radius: float
    | Square of side: float
    | Triangle of height: float * width: float

Fails in FSI

[<Struct>]
type StructShape =
    | Circle of radius: float
    | Square of side: float
    | Triangle of height: float * width: float

[KevinRansom]

@cartermp

The compiler doesn't like it either:

// Learn more about F# at http://fsharp.org
// See the 'F# Tutorial' project for more help.

[<Struct>]
type StructShape =
    | Circle of radius: float
    | Square of side: float
    | Triangle of height: float * width: float

[<EntryPoint>]
let main argv = 
    printfn "%A" argv
    0 // return an integer exit code

Fails with:

1>FSC: error FS2014: A problem occurred writing the binary 'C:\Users\kevinr\Documents\Visual Studio 2017\Projects\ConsoleApplication31\ConsoleApplication31\obj\Debug\ConsoleApplication31.exe': Error in pass2 for type Program, error: Error in pass2 for type StructShape, error: duplicate entry '.ctor' in method table
1>Done building project "ConsoleApplication31.fsproj" -- FAILED.

[cartermp]

Same underlying issue as #1678 then. Bah.

[dsyme]

Yes, my apologies for #1678. We will address it in F# 4.2 (or next update of F# 4.1) For now we have to list it as a known limitation.

[dsyme]

I'll close this as a duplicate of #1678