Proposal: Negative Pattern

[alrz]

Moved from: #16742 (comment)

I propose to add a pattern operator to negate the effect of another pattern,

not-pattern:
 not pattern

A "negative pattern" could be applied to an arbitrary pattern and reverse the failure path.

Examples:

if (obj is not null)

switch (obj) {
  case not Constant:
      break;
}

It could be even nested in recursive patterns as well,

case (not 0, var y):
// equivalent to
case (var x, var y) when x != 0:

This would be consistent with other proposed pattern operators like #6235.

Note that definitive assignment rules are reversed, for example,

if (obj is not T t) return;
// t is definitely assigned

This also eliminates the need to parenthesize the is expression in case you want to negate the result.

[gordanr]

Very interesting. Can you give more examples, please.

[alrz]

To clarify, this is just another "recursive pattern", once those are introduced to the language you can imagine more usages from combining various kinds of patterns.

[Thaina]

is! and case! should just be valid syntax

Recursive pattern should be just as it is

[alrz]

Moved to dotnet/csharplang#246.