Fsharp: Single case struct discriminated union with the same case name as the type silently converted to object type

[zpodlovics]

Please provide a succinct description of the issue.

Repro steps

Provide the steps required to reproduce the problem

1. Create a new fsharp console project

dotnet new console -lang F#

1. Copy the following content to Program.fs

/// Author: Zoltan Podlovics
/// License: Apache License, Version 2.0, https://www.apache.org/licenses/
/// Description: Single case struct discriminated union with the same case 
/// name as the type silently converted to object type

open System

[<Struct>]
type StructDUSameName = StructDUSameName

[<Struct>]
type StructDUSameNamePipe = | StructDUSameNamePipe

[<Struct>]
type StructDUSameNameNewLine = 
    | StructDUSameNameNewLine

[<Struct>]
type StructDUIntSameName = StructDUIntSameName of int

[<Struct>]
type StructDUIntSameNamePipe = | StructDUIntSameNamePipe of int

[<Struct>]
type StructDUIntSameNameNewLine = 
    | StructDUIntSameNameNewLine of int

[<EntryPoint>]
let main argv =
    printfn "sizeof<StructDUSameName>: %d" sizeof<StructDUSameName>
    printfn "typeof<StructDUSameName>.IsValueType: %b" typeof<StructDUSameName>.IsValueType
    printfn "sizeof<StructDUSameNamePipe>: %d" sizeof<StructDUSameNamePipe>
    printfn "typeof<StructDUSameNamePipe>.IsValueType: %b" typeof<StructDUSameNamePipe>.IsValueType
    printfn "sizeof<StructDUSameNameNewLine>: %d" sizeof<StructDUSameNameNewLine>
    printfn "typeof<StructDUSameNameNewLine>.IsValueType: %b" typeof<StructDUSameNameNewLine>.IsValueType
    printfn "sizeof<StructDUIntSameName>: %d" sizeof<StructDUIntSameName>
    printfn "typeof<StructDUIntSameName>.IsValueType: %b" typeof<StructDUIntSameName>.IsValueType
    printfn "sizeof<StructDUIntSameNamePipe>: %d" sizeof<StructDUIntSameNamePipe>
    printfn "typeof<StructDUIntSameNamePipe>.IsValueType: %b" typeof<StructDUIntSameNamePipe>.IsValueType
    printfn "sizeof<StructDUIntSameNameNewLine>: %d" sizeof<StructDUIntSameNameNewLine>
    printfn "typeof<StructDUIntSameNameNewLine>.IsValueType: %b" typeof<StructDUIntSameNameNewLine>.IsValueType

    0 // return an integer exit code

1. Execute with .NET Core 2.0

dotnet run -c Release

Execution result:

sizeof<StructDUSameName>: 8
typeof<StructDUSameName>.IsValueType: false
sizeof<StructDUSameNamePipe>: 1
typeof<StructDUSameNamePipe>.IsValueType: true
sizeof<StructDUSameNameNewLine>: 1
typeof<StructDUSameNameNewLine>.IsValueType: true
sizeof<StructDUIntSameName>: 4
typeof<StructDUIntSameName>.IsValueType: true
sizeof<StructDUIntSameNamePipe>: 4
typeof<StructDUIntSameNamePipe>.IsValueType: true
sizeof<StructDUIntSameNameNewLine>: 4
typeof<StructDUIntSameNameNewLine>.IsValueType: true

1. Compile to Mono 5.2.x

fsharpc --optimize+ Program.fs

1. Execute with Mono 5.2.x

mono Program.exe

Execution result:

sizeof<StructDUSameName>: 8
typeof<StructDUSameName>.IsValueType: false
sizeof<StructDUSameNamePipe>: 0
typeof<StructDUSameNamePipe>.IsValueType: true
sizeof<StructDUSameNameNewLine>: 0
typeof<StructDUSameNameNewLine>.IsValueType: true
sizeof<StructDUIntSameName>: 4
typeof<StructDUIntSameName>.IsValueType: true
sizeof<StructDUIntSameNamePipe>: 4
typeof<StructDUIntSameNamePipe>.IsValueType: true
sizeof<StructDUIntSameNameNewLine>: 4
typeof<StructDUIntSameNameNewLine>.IsValueType: true

structdusamename.zip

Expected behavior

Struct discriminated union not converted silently to object (non-value) type.

Actual behavior

Single case struct discriminated union with the same case name as the type silently converted to object type. Any idea why the single case struct discriminated union size is different with .NET Core vs Mono?

Known workarounds

Use the pipe character | before the discriminated union case name.

Related information

• Operating system: Ubuntu 16.04 x86_64
• Branch: xenial
• .NET Runtime, CoreCLR or Mono Version:

.NET Core 2.0 (production)

Mono JIT compiler version 5.2.0.215 (tarball Mon Aug 14 15:46:23 UTC 2017)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
    TLS:           __thread
    SIGSEGV:       altstack
    Notifications: epoll
    Architecture:  amd64
    Disabled:      none
    Misc:          softdebug 
    LLVM:          supported, not enabled.
    GC:            sgen (concurrent by default)

[abelbraaksma]

An intriguing report. I just ran it partially on VS2017 15.3.1.0 Preview 1 with latest nightlies on Windows 7 from FSI and I get similar peculiarities:

> [<Struct>] type S = S;;
[<Struct>]
type S = | S

> sizeof<S>;;
val it : int = 8

> [<Struct>] type S = | S;;
[<Struct>]
type S = | S

> sizeof<S>;;
val it : int = 1

> [<Struct>] type S = Y;;
[<Struct>]
type S = | Y

> sizeof<S>;;
val it : int = 8

Which also shows that it doesn't matter whether it is the same name, it seems to matter whether or not the pipeline character is there or not. Notice, however, that the type is recognized as the same, it just isn't stored the same.

[zpodlovics]

@abelbraaksma you are right, I missed that non-same name case somehow, and the my conclusion was not fully precise.

Intrestingly the F# Interactive (wrongly) will print the as [<Struct>] type description after evaluation, however if you really check the types with typeof<MyType> it will have different BaseClass (System.Object vs System.ValueType) and IsValueType property (false vs true).

> type ClassDUSameName = ClassDUSameName;;             
type ClassDUSameName = | ClassDUSameName

> type ClassDUSameName = | ClassDUSameName;;
type ClassDUSameName = | ClassDUSameName

> type ClassDU = ClassDUCase1;;             
type ClassDU = | ClassDUCase1

> type ClassDU = | ClassDUCase1;;
type ClassDU = | ClassDUCase1

> type [<Struct>] StructDUSameName1 = StructDUSameName1;;
[<Struct>]
type StructDUSameName1 = | StructDUSameName1

> typeof<StructDUSameName1>.IsValueType;;                
val it : bool = false

> type [<Struct>] StructDU1 = StructDU1Case1;;  
[<Struct>]
type StructDU1 = | StructDU1Case1

> typeof<StructDU1>.IsValueType;;             
val it : bool = false

[abelbraaksma]

@cartermp: can we label this with high priority? The analysis by @zpodlovics shows that structs are not always structs, meaning they end up on the heap. This has potentially serious effects and is directly counter to the syntax used.

Also, it may be a good idea to look into some types returning 0 for their size? Or should that be treated as a different issue? Sounds like a bug in the sizeof operator to me.

[zpodlovics]

@abelbraaksma a more reasonable check would be to check whether the type is a value type or not.

The empty struct size seems to be one (1) on .NET Framework and .NET Core, as the sizeof returns the runtime representation size (and not the intended struct size). Mono was able somehow to provide zero sized empty structs, probably it's related to their shared value type representation (http://www.mono-project.com/docs/advanced/runtime/docs/gsharedvt/ and http://www.mono-project.com/docs/advanced/runtime/docs/generic-sharing/).

Please note, no mandatory field needed to have a correct non-zero sized runtime representation. I did a quick last day, the C# compiler in .NET Core 2.0 is perfectly happy with empty structs. Nested empty structures are also pefectly fine. No complaints (not even a warning) from the compiler.

Here is how to check a zero sized struct (probably there are better solutions now):
https://stackoverflow.com/questions/16611598/why-does-an-empty-struct-in-c-sharp-consume-memory

[dsyme]

@zpodlovics @abelbraaksma I pushed a proposed fix. Thanks so much for your careful checking of many cases

[dsyme]

Please note, no mandatory field needed to have a correct non-zero sized runtime representation.

Just to mention that without a field or size attribute peverify gives

[MD]: Error: Value class has neither fields nor size parameter. [token:0x0200000F]
[MD]: Error: Value class has neither fields nor size parameter. [token:0x02000011]

[dsyme]

Fixed in #3521

[abelbraaksma]

Great work, @dsyme, tx. I'll have a look at this once the new nightly comes around (they seem to not be automatically build anymore?).

[dsyme]

once the new nightly comes around (they seem to not be automatically build anymore?).

There was a glitch in the build, should be fixed by #3528