PM design question: Scopes

[Brian Goetz]

On 11/20/2017 1:33 PM, Guy Steele wrote:
> I like this.  One question: what does this new theory have to say 
> about the situation
>
> switch (x) {
>   case Foo(int x):
> int y = x;
> // fall through
>   case Bar(int x, int y):
> …
> }

In this case, I would say that the second y is shadowing the first, and 
therefore this is an error.  Trying to merge the ys seems like a heroic 
measure.  Merging the xs, on the other hand, is clean, because at the 
point where the second x is bound, the first x is DU (we'd skip over the 
Bar(int x, int y) binding if we had matched the first case.)

The opposite example is also interesting:

     case Foo(int x, int y):
         // A
         // fall through
     case Bar(int x):
         // B

Here, y is in scope in A, but not in B; x is in scope in both A and B.

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