Missing FS0750 in nested let bang (got lost between 9.0 and 10.0)

[Martin521]

A let! nested in a let binding used to produce error FS0750: This construct may only be used within computation expressions (in all SDKs from at least 5.0 up to 9.0), but no more in 10.0.

Example:

let y() =   // unit -> Task<Task<string array>>
    task {
        let a =
            let! b = System.IO.File.ReadAllLinesAsync("")
            b
        return a
    }

The let! in the fourth line is silently interpreted as a simple let, as you can see from the types of a, b and y.

(I do actually believe it should be properly compiled, but that is a separate issue. The first step is probably to bring the error message back.)

[github-actions]

🤖 This is an automated response from Repo Assist.

The regression (missing FS0750 for nested let! in a plain let binding inside a task CE) appears to be fixed in the development build on main.

Verification with fsi 15.2.100.0 for F# 11.0 (commit a0cce49):

$ dotnet fsi test19456.fsx
/tmp/test19456.fsx(4,13): error FS0750: This construct may only be used within computation expressions

The repro:

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

Context:

• F# 9.0 and earlier: correctly raised FS0750 ✅
• F# 10.0 (SDK 10.0.105): silently compiled let! as let, making a : Task<string[]> and y() : Task<Task<string[]>> ❌
• F# 11.0 dev (main, fsi 15.2.100.0): correctly raises FS0750 again ✅

The fix for the missing FS0750 is present in main. Issue #19457 (whether nested let! should actually compile successfully) is a separate design question.

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