Treat span-like types with a unit of measure as byref-like

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
40/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Ambito
compilers

Direzione di ricerca

Start from isByrefLikeTyconRef and stripTyEqns in the compiler's type utilities, which the report says miss the representation of measure-annotated abbreviations. Run the repro with the TaggedSpan<kg> example and compare with the TaggedSpan<1> case, which already gives FS0406 and FS0412. Done means the kg version reports the same errors and existing byref-like tests still pass. The author already has a fix, so check with the maintainers before starting a parallel one.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Needs-Triage

A measure-annotated abbreviation of Span<int> is not treated as byref-like when its unit of measure is anything other than 1. The compiler then accepts code that captures such a value in a closure or uses its type as a type argument, and the program fails at run time.

open System
[<Measure>] type kg
[<MeasureAnnotatedAbbreviation>]
type TaggedSpan<[<Measure>] 'u> = Span<int>
let size (s: TaggedSpan<'u>) = 0

let capture (s: TaggedSpan<kg>) = let t = s in fun () -> size t
let instantiate (s: TaggedSpan<kg>) = id s

[<EntryPoint>]
let main args =
    let s = Unchecked.defaultof<TaggedSpan<kg>>
    if args.Length = 0 then capture s () else size (instantiate s)

Actual: this compiles with the .NET SDK 10.0.402 compiler and with main at fbc9be3. Compiled without optimization:

  • With no arguments, capture throws TypeLoadException: A ByRef or ByRef-like type cannot be used as the type for an instance field in a non-ByRef-like type.
  • With an argument, instantiate throws VerificationException: Method Microsoft.FSharp.Core.Operators.Identity: type argument 'System.Span`1[System.Int32]' violates the constraint of type parameter 'T'.

Expected: the errors the compiler reports when kg is replaced by 1: FS0406 on the closure in capture, and FS0412 on id s and on Unchecked.defaultof.

isByrefLikeTyconRef does not look at the representation of a measure-annotated type, and stripTyEqns reduces such a type to its representation only when the measure is dimensionless, so TaggedSpan<kg> is not recognised as byref-like. I found this while reviewing #20856, whose tests define TaggedSpan. I have a fix ready.

[!NOTE]
Prepared with AI assistance (Anthropic Claude, OpenAI Codex); Peter reviewed it.

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