[patterns] Nullability in patterns, and pattern-aware constructs (again)

[Brian Goetz]

I guess that means the stuff I said about "only having to look in two 
places" for null-handling was wrong; any static (declared) pattern could 
be nullable.  Which brings us back to the place that people didn't like 
the last time around -- you have to scrutinize the implementations of 
the patterns associated with all N cases to determine whether this 
switch will take nulls or not.

On 1/9/2020 7:57 PM, John Rose wrote:
>
> We haven’t yet moved on to library-defined patterns (“static
> patterns”).  To gaze a bit into the crystal ball:  I think it’s
> likely that those will provide a way to express T? using
> a library API instead of a language primitive.  We should
> defer building out that last 1% of null support either
> indefinitely, or until there is a way for libraries to define
> their own kinds of patterns.  In the latter case the cost
> of adding a nullable pattern is likely to be lower than
> the alternatives on the table, since library changes are
> always cheaper than language changes.

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