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FS-1053 update :: Support instantiation of C#-defined "allows ref struct" generics with ByrefLike types #787

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Nov 5, 2024
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FS-1053 update :: Support instantiation of C#-defined "allows ref struct" generics with ByrefLike types #787

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2 changes: 2 additions & 0 deletions FSharp-4.5/FS-1053-span.md
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Expand Up @@ -336,6 +336,8 @@ in some situations where capture must be avoided.
* Cannot be static or instance members of a class or normal struct
* Cannot be captured by any closure construct (async methods or lambda expressions)
* Cannot be used as a generic parameter
- In F# 9 and higher, this restriction is relaxed if the generic parameter is defined in C# using the `allows ref struct` anti-constraint. F# can instantiate such generics in types and methods with byref-like types. As a few examples, this affects BCL delegate types (`Action<>`,`Func<>`), interfaces (`IEnumerable<>`,`IComparable<>`) and generic arguments with a user-provided accumulator function (`String.string Create<TState>(int length, TState state, SpanAction<char, TState> action)`)
- It is impossible to author generic code supporting byref-like types in F#.
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@brianrourkeboll brianrourkeboll Aug 28, 2024

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Would it be worth expanding on this here? E.g., to mention that the allows ref struct anti-constraint is not automatically propagated in the same way as normal generic constraints, with some examples.

let a : Action<'T> = …
let f (x : 'T) = a.Invoke x // Can 'T be ReadOnlySpan<int> here? Presumably not?
let f (g : Action<'T>) (x : 'T) = g.Invoke x // Can 'T be ReadOnlySpan<int> here?

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@T-Gro T-Gro Aug 29, 2024

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It is not automatically propagated, this makes sense to expand on - thanks for checking this.


Here is an example:
```fsharp
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