Computation Expressions bug when using MaintainsVariableSpace / ProjectionParameter

```fsharp
type AH =
| AH

member _.For(x, f) = f x

member _.Yield x = x

[]
member _.foo(x, []y) =
let x = y x
x, "A"

[]
member _.select(src, []f) = f src

let foo (x: int) =
AH {
for x in x do
let n = x
foo (n + x)
select n
}
```
The above code compiles successfully, but the result is not the value of `n`, as intuitively expected, but `"A"`. I understand why this is the case, but can we be smarter when parsing the CE?

Because if you don't consider the implementation of CE, it is easy for users to write wrong expressions.

But if the CE implementation considers how to avoid this, there is only one conclusion: do not generate tuples.

Not generating tuples often makes some code impossible to optimize.

Start by compiling the provided F# computation-expression example and confirming that it produces "A" rather than the expected value of n. Investigate how MaintainsVariableSpace and ProjectionParameter are parsed, including the tuple-generation trade-off. Done means the behavior is resolved or clearly specified without unnecessarily preventing optimization.

Written by the indexing model from the issue text.