Invalid type abbreviation delcaration allowed

```fsharp
type Oops = Span<int> * Span<int>

let f (x: Oops) = ()

let failsAsIntended (x: Span<int> * Span<int>) = ()
```

`failsAsIntended` will fail to compile, since you can't tuple a Span. But `f` and the `Oops` declaration do compile.