在我发表 comments https://github.com/fsharp/fslang-suggestions/issues/349#issuecomment-1124206512

为方便读者，此处复制了示例代码:

let spanOfOptString(os: Option<string>) =
  match os with Some(s) -> s.AsSpan()
                | None -> raise(Exception())

错误FS0412:类型实例化涉及byref类型.这是通用IL规则所不允许的.

因此，F#将一个无返回函数raise键入为Exception -> 'a，其中包含一个free 'a("OCaml方式")，而类型变量恰好绑定到byref类型ReadOnlySpan，这是非法的.我觉得我是因为我没有犯下的罪行而受到惩罚...

我可以找到一个解决办法

let checkedSpanOfString(os: Option<string>) =
  match os with Some(s) -> s.AsSpan()
                | None -> raise(Exception())  // 'a = unit
                          ReadOnlySpan.Empty  // both arms: ReadOnlySpan<char>

https://sharplab.io/#gist:32b520574fde97de8d7389ab04f64bc4

但我觉得这有点难看.我认为我们不应该期望all个byref类型可以使用.Empty个？(我想除了(ReadOnly)Span/Memory之外，我从来没有使用过任何其他byref类型，但那是另一个故事)

While F# core team doesn't seem to be moving towards introducing the bottom type to the language (which is perhaps a sensible engineering decision),
does anybody have a better alternative to make this work?

Update

@chadnt的答案可以内联以获得

let checkedSpanOfString(os: Option<string>) =
  let s = match os with Some(s) -> s
                        | None -> raise(Exception())
  s.AsSpan()

https://sharplab.io/#gist:a5eab805c539c45048b4072fa7b096c5

它的IL看起来比我的原始版本简单得多