Bug in byref-return checking: passing local struct address should not be permitted

[dsyme]

This is a bug in ref-return safety checking, a missing error for negative test case https://github.com/Microsoft/visualfsharp/pull/1367/files#diff-59ae1d2137d6b7479475f5d1287557d9L13

For most cases of returning byrefs we correctly report an error, e,g

let negTest5() = 
    let mutable x = 0
    let addr = &x
    &addr

But not for this case:

Repro steps

Compile this code:

module Test

[<Struct>]
type Struct(initial:int) = 
    let mutable x = initial
    member __.AddrX = &x
    member __.IncrX() = x <- x + 1

let negTest7() = 
    let mutable iterator = Struct(0)
    iterator.AddrX

Expected behavior

We expect an error to be reported

Actual behavior

No error is reported

[forki]

nice one ;-)

[dsyme]

Closing in favour of the bucket bug #862