Correct StructLayout size emission for data-less struct unions

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,11 +1,11 @@
 ### Fixed
 
+* Fix incorrect `StructLayout(Size = 1)` emission for data-less struct unions where the compiler-generated tag field makes the actual runtime size larger. ([PR #19759](https://github.com/dotnet/fsharp/pull/19759))
 * Tooltip "Full name" now shows demangled companion module names (e.g. `MyType.func` instead of `MyTypeModule.func`). ([Issue #17335](https://github.com/dotnet/fsharp/issues/17335), [PR #19867](https://github.com/dotnet/fsharp/pull/19867))
 * 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))
 * Fix `TypeLoadException` ("Specialize tried to implicitly override a method with weaker type parameter constraints") and the related CLR crash with constrained inline calls by stripping constraints from closure-class typars in `EraseClosures.convIlxClosureDef`. ([Issue #14492](https://github.com/dotnet/fsharp/issues/14492), [Issue #19075](https://github.com/dotnet/fsharp/issues/19075), [PR #19882](https://github.com/dotnet/fsharp/pull/19882))
-
 * Suppress hover/symbol resolution for wildcard `_` patterns inside `member _.…` bodies that incorrectly showed `val _: T` tooltip. ([PR #19760](https://github.com/dotnet/fsharp/pull/19760))
 * Deduplicate format specifier locations in computation expressions so editor tooling no longer reports duplicate entries for the same `%` specifier. ([Issue #16419](https://github.com/dotnet/fsharp/issues/16419), [PR #19791](https://github.com/dotnet/fsharp/pull/19791))
 * Reject non-function bindings for single-case and partial active pattern names with FS1209, matching the existing multi-case behavior. ([PR #19763](https://github.com/dotnet/fsharp/pull/19763))

src/Compiler/CodeGen/IlxGen.fs

@@ -12118,18 +12118,10 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                         }
 
                     let layout =
-                        // Structs with no instance fields get size 1, pack 0
+                        // Multi-case struct unions carry a hidden tag field; single-case struct unions
+                        // are handled by the CLR's minimum-1-byte guarantee. No explicit size needed.
                         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
-                                ILTypeDefLayout.Sequential { Size = None; Pack = None }
-                            else
-                                ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }
+                            ILTypeDefLayout.Sequential { Size = None; Pack = None }
                         else
                             ILTypeDefLayout.Auto
 

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs

@@ -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 multi-case struct DUs with no instance fields`` () =
         Fsx """
         [<Struct>] type Option<'T> = None | Some
         """
@@ -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 )                   
@@ -426,4 +424,28 @@ module CustomAttributes_Basic =
       .field public static literal int32 Some = int32(0x00000001)
     } 
         """
-        ]
+        ]
+
+    [<Fact>]
+    let ``StructLayoutAttribute doesn't have size=1 for single-case struct DU`` () =
+        Fsx """
+        [<Struct>] type X = | Y
+        """
+        |> compile
+        |> shouldSucceed
+        |> verifyIL [
+        """
+        .class sequential autochar serializable sealed nested public beforefieldinit X
+        extends [runtime]System.ValueType
+        implements class [runtime]System.IEquatable`1<valuetype Test/X>,
+                   [runtime]System.Collections.IStructuralEquatable,
+                   class [runtime]System.IComparable`1<valuetype Test/X>,
+                   [runtime]System.IComparable,
+                   [runtime]System.Collections.IStructuralComparable
+  {
+    .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 )                   
+    .custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationMappingAttribute::.ctor(valuetype [FSharp.Core]Microsoft.FSharp.Core.SourceConstructFlags) = ( 01 00 01 00 00 00 00 00 ) 
+        """
+        ]