Wrong StructLayoutAttribute.Size for struct unions with no data fields

[ghost]

Consider this type:

[<Struct>]
type ABC = A | B | C

F# 9 compiles it to a struct with this attribute (among others):

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Size = 1)]

However, because of the compiler-generated _tag field, the size is actually 4:

> sizeof<ABC>;;                                                        
val it: int = 4

This seems not to cause any problems, but the documentation states "This field must be equal or greater than the total size, in bytes, of the members of the class or structure," which suggests this should be corrected.

[vzarytovskii]

It's probably some flaw in logic around:

fsharp/src/Compiler/CodeGen/IlxGen.fs

Lines 11567 to 11576 in 935b796

	if
	tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic)
	||
	// Reflection emit doesn't let us emit 'pack' and 'size' for generic structs.
	// In that case we generate a dummy field instead
	(cenv.options.workAroundReflectionEmitBugs && not tycon.TyparsNoRange.IsEmpty)
	then
	ILTypeDefLayout.Sequential { Size = None; Pack = None }, ILDefaultPInvokeEncoding.Ansi
	else
	ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }, ILDefaultPInvokeEncoding.Ansi

Should be relatively easy to investigate and test. Should be a good first issue, gonna mark it like this.

[timofey256]

Hello, I would like to try fix this.

1. What would be the expected Size for the struct would be? I see a few lines below that GenTypeDef specifies it as None. Is this the expected behavior for value type DUs too?
2. If so, would something like this work?

if
    tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic)
    ||
    // Reflection emit doesn't let us emit 'pack' and 'size' for generic structs.
    // In that case we generate a dummy field instead
    (cenv.options.workAroundReflectionEmitBugs && not tycon.TyparsNoRange.IsEmpty)
    ||
    **// Discriminated unions have a compiler-generated _tag field even when they don't have any data fields,
    // so we should not explicitly set Size = 1 for them
    (match tycon.TypeReprInfo with
     | TFSharpTyconRepr { fsobjmodel_kind = TFSharpUnion } -> true
     | _ -> false)**
then
    ILTypeDefLayout.Sequential { Size = None; Pack = None }, ILDefaultPInvokeEncoding.Ansi
else
    ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }, ILDefaultPInvokeEncoding.Ansi

3. Where are tests for this part of code? Unfortunately, couldn't locate.

[vzarytovskii]

This would work, if the goal is to not set the size.

You shouldn't match tycon directly, it should have IsUnionTycon tester.

Tests can be added independently to the component tests suite.