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Correct StructLayout size emission for data-less struct unions #19759

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Correct StructLayout size emission for data-less struct unions #19759

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Copilot May 18, 2026
7384c60
Fix struct union StructLayout size emission
Copilot May 18, 2026
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10 changes: 2 additions & 8 deletions src/Compiler/CodeGen/IlxGen.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -12031,15 +12031,9 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
}

let layout =
// Structs with no instance fields get size 1, pack 0
// Struct unions always carry a hidden tag field, so never emit size 1 here
if isStructTy g thisTy then
if
(tycon.AllFieldsArray.Length = 0
|| tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic))
&& (alternatives
|> Array.collect (fun a -> a.FieldDefs)
|> Array.exists (fun fd -> not fd.ILField.IsStatic))
then
if tycon.IsUnionTycon then
ILTypeDefLayout.Sequential { Size = None; Pack = None }
else
ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }
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6 changes: 2 additions & 4 deletions ....Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs
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Expand Up @@ -397,7 +397,7 @@ module CustomAttributes_Basic =
]

[<Fact>]
let ``StructLayoutAttribute has size=1 for struct DUs with no instance fields`` () =
let ``StructLayoutAttribute doesn't have size=1 for struct DUs with no instance fields`` () =
Fsx """
[<Struct>] type Option<'T> = None | Some
"""
Expand All @@ -413,8 +413,6 @@ module CustomAttributes_Basic =
[runtime]System.IComparable,
[runtime]System.Collections.IStructuralComparable
{
.pack 0
.size 1
.custom instance void [FSharp.Core]Microsoft.FSharp.Core.StructAttribute::.ctor() = ( 01 00 00 00 )
.custom instance void [runtime]System.Diagnostics.DebuggerDisplayAttribute::.ctor(string) = ( 01 00 15 7B 5F 5F 44 65 62 75 67 44 69 73 70 6C
61 79 28 29 2C 6E 71 7D 00 00 )
Expand All @@ -426,4 +424,4 @@ module CustomAttributes_Basic =
.field public static literal int32 Some = int32(0x00000001)
}
"""
]
]
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