Matching nullables with var pattern

[alrz]

Version Used: features/recursive-patterns

Steps to Reproduce:

using System;
class C {
    extern static (int, int)? Get();   

    static void Test() {
        if (Get() is var (x, y)) {
            Console.Write(x);   
            Console.Write(y);   
        }
    }
}

Expected Behavior: Should check for HasValue and deconstruct.

Actual Behavior: Error.

Note: should work for any deconstructible, not just tuples.
Note: in effect, this should be equivalent to the (var x, var y) pattern.

[Joe4evr]

Note: in effect, this should be equivalent to the (var x, var y) pattern.

I'm not so sure, the var pattern is supposed to match everything, regardless of the value. Once we get those coveted Property patterns, this might naturally fall out of using the Empty Property pattern as a null-check:

if (Get() is {} (x, y))
{
    //code....
}

[alrz]

the var pattern is supposed to match everything,

A var-pattern with a parenthesized designation cannot possibly match everything. Now, the current behavior comes from deconstruction assignment var (x, y) = e; (you can't deconstruct a nullable tuple). But since here we're talking about patterns (rather than mere deconstruction), we can actually fail the match on null.

if (Get() is {} (x, y))

First off, this is an invalid pattern, unless x and y are constants - in which case the correct syntax is
(x, y) {}. If you want to introduce variables you should use var x instead. The identifiers alone only work in a var-pattern, just like the example above.

Also you don't need to do {} when you're matching with a deconstruction pattern,

if (Get() is (var x, var y))

This check for null (if needed) and deconstruct into variables.

I believe the same behavior is expected with the shorthand form var (x, y).

[gafter]

This is definitely a bug. It should simply work.

[gafter]

It should work for pattern-matching. It should not work with deconstruction.

[alrz]

I think new object() is var (x, y) should match with ITuple, I suspect it would have the same fix.

[gafter]

Yes, new object() is var (x, y) should yield false.

[gafter]

Fixed in #31056