Repro:

1. Open VisualFSharp.sln
2. Open SignatureHelp.fs
3. Put a breakoint on line 123(let startOfArgs = ...)
4. Launch a hive of VS where you can debug
5. Enter the following source code:

type C() =
    static member M(x, y, z)  = ()

C.M(1)

6. Place cursor after the 1 on the last line
7. Press the comma character (,) so that Signature Help is triggered

Note that there are some locals, and critically you can observe the value of applicableSpan:

Now try to get the source code that matches this span by:

1. Opening the Immediate Window
2. Entering the following:

sourceText.GetSubText(applicableSpan)

You will observe that sourceText doesn't exist. Indeed, it wasn't even in the locals window.

However, note that sourceText is a parameter to this method and it is clearly accessible.

We are losing debug info on every do! call in the method.

To test this, place a breakpoint on line 89 (do! Option.guard (methods.Length > 0 ...). When it is broken there, you'll see that sourceText is available: