Nested let! should compile

[Martin521]

A let! nested in a simple let binding is currently not allowed by the compiler.

Example:

let y() =
    task {
        let a =
            let! b = System.IO.File.ReadAllLinesAsync("")
            b
        return a
    }

The 4th line produces error FS0750: This construct may only be used within computation expressions.
(Except for SDK 10.0, where the let! is silently compiled as a simple let, see #19456.)

I believe this behavior is a bug and the above example should compile correctly.

The spec, as quoted here, prescribes the following transformations:

{{ let binding in cexpr }} => let binding in {{ cexpr }} and
{{ let! pattern = expr in cexpr }} => builder.Bind(expr, (fun pattern -> {{ cexpr }}))

It seems that applying them to the example code is straightforward and should lead to correct compilation of the CE rather than to an error message.