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Fix StructLayout Size for struct discriminated unions #19946

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Fix StructLayout Size for struct discriminated unions #19946

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7dcdb8c
Fix StructLayout Size for struct discriminated unions
KirtiRamchandani Jun 14, 2026
ef9374e
Add release notes for struct union StructLayout Size fix
KirtiRamchandani Jun 15, 2026
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1 change: 1 addition & 0 deletions docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
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@@ -1,5 +1,6 @@
### Fixed

* Fix `StructLayout` `Size` for struct discriminated unions so emitted IL matches the computed layout size. ([Issue #18125](https://github.com/dotnet/fsharp/issues/18125), [PR #19946](https://github.com/dotnet/fsharp/pull/19946))
* Fix internal error (FS0193) when calling an indexed property setter with a named argument that matches an indexer parameter. ([Issue #16034](https://github.com/dotnet/fsharp/issues/16034), [PR #19851](https://github.com/dotnet/fsharp/pull/19851))
* Fix missing FS1182 ("unused binding") warning for unused `let` function bindings inside class types. ([Issue #13849](https://github.com/dotnet/fsharp/issues/13849), [PR #19805](https://github.com/dotnet/fsharp/pull/19805))
* Fix inner mutually-recursive `let rec ... and ...` functions under `--realsig+` not being lifted to top-level static methods (TLR), causing `FSharpFunc` closure allocations and loss of `tail.` opcodes — the large struct-mutual-recursion perf regression reported in [Issue #17607](https://github.com/dotnet/fsharp/issues/17607). ([PR #19882](https://github.com/dotnet/fsharp/pull/19882))
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1 change: 1 addition & 0 deletions src/Compiler/CodeGen/IlxGen.fs
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Expand Up @@ -12019,6 +12019,7 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
// Structs with no instance fields get size 1, pack 0
if
tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic)
|| tycon.IsUnionTycon

@T-Gro T-Gro Jun 24, 2026

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This code path is for objects and records, union IL codegen happens elsewhere.
I am going to close this PR and continue at #19759 , but please do come over and share feedback there, I am happy to improve it even further 👍 .

@KirtiRamchandani KirtiRamchandani Jun 24, 2026

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Thanks, it looks like you were already working there on the same issue. I just posted comment there.

||
// Reflection emit doesn't let us emit 'pack' and 'size' for generic structs.
// In that case we generate a dummy field instead
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12 changes: 12 additions & 0 deletions tests/FSharp.Compiler.ComponentTests/EmittedIL/Structure/StructUnionLayout18125.fs
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@@ -0,0 +1,12 @@
// Regression test for https://github.com/dotnet/fsharp/issues/18125
module StructUnionLayout18125

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

let verifySize () =
// Struct DU has a compiler-generated _tag field; sizeof must not be 1.
if sizeof<ABC> <> 4 then
failwith $"Expected sizeof<ABC> = 4, got {sizeof<ABC>}"

verifySize ()
7 changes: 7 additions & 0 deletions tests/FSharp.Compiler.ComponentTests/EmittedIL/Structure/Structure.fs
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Expand Up @@ -235,3 +235,10 @@ module Structure =
compilation
|> getCompilation
|> verifyExecution

// SOURCE=StructUnionLayout18125.fs SCFLAGS="-r:CodeGenHelper.dll" # StructUnionLayout18125.fs
[<Theory; FileInlineData("StructUnionLayout18125.fs")>]
let ``StructUnionLayout18125_fs`` compilation =
compilation
|> getCompilation
|> verifyExecution
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