Correct StructLayout size emission for data-less struct unions

[Copilot]

Struct unions with no declared payload fields were emitted with StructLayout(Size = 1), even though the compiler-generated tag field makes the actual runtime size larger. This updates union codegen so data-less struct unions no longer advertise an invalid explicit size.

• Codegen

  • Adjust the struct-union layout path in IlxGen.fs to stop treating struct unions as empty structs.
  • For struct unions, emit sequential layout without forcing Size = 1, allowing the generated tag field to determine the effective size.

• Regression coverage

  • Update the existing component test for data-less struct unions to assert that the emitted IL no longer contains:

    .pack 0
    .size 1
    

  • Keep the existing coverage for struct unions with instance fields unchanged.

• Example

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

  Before, this could be emitted with StructLayout(Size = 1). With this change, the compiler no longer emits that incorrect explicit size for the struct union.

[T-Gro]

Review: Correct StructLayout size emission for data-less struct unions

The fix is correct for the motivating scenario — multi-case all-nullary struct unions (e.g., [<Struct>] type ABC = A | B | C) were incorrectly emitted with .size 1 even though the compiler generates a _tag field that makes them non-empty. Removing the explicit size is the right call.

Issues to address

1. Dead else branch (bug)

The condition at line 12036:
sharp if tycon.IsUnionTycon then ILTypeDefLayout.Sequential { Size = None; Pack = None } else ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }

This code lives inside the TFSharpUnion match arm (line 12000–12003), so tycon.IsUnionTycon is always true here. The else branch is unreachable dead code.

The intent should be simplified to:
sharp let layout = if isStructTy g thisTy then ILTypeDefLayout.Sequential { Size = None; Pack = None } else ILTypeDefLayout.Auto

2. Inaccurate comment

// Struct unions always carry a hidden tag field, so never emit size 1 here

This is not true for single-case struct unions (e.g., [<Struct>] type Foo = | Bar). Those use SingleCaseStruct layout which has NoTagField — no _tag field is emitted. The fix still works correctly for that case (CLR guarantees ≥1 byte for value types without explicit size), but the comment misleads future readers.

Suggested:
sharp // 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.

Minor

• The test rename is good. Asserting that .pack 0 / .size 1 are absent properly validates the fix.
• Consider adding a single-case struct union ([<Struct>] type X = | Y) to the test suite to document behavior in that edge case, since the original Size=1 annotation is being removed for all struct unions, not just multi-case ones.

[github-actions]

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