Add support for constant pattern matching assignments

[mburbea]

Consider this snippet:

Consider the following snippet::

if (span.IndexOf(pattern) is {} x and -1)
{
     // bail not found
     return;
}
Console.WriteLine(x); // CS0165: use of unassigned local variable x.

And the seemingly similar code:

if (span.IndexOf(pattern) is {} x && x==-1)
{
    return;
}
Console.WriteLine(x); // No error

I propose we allow variable assignment after any pattern so that this code in theory could be rewritten to do something like:

if (span.IndexOf(pattern) is not -1 x)
{
   Console.WriteLine(x);  
}

[333fred]

Is that (not - 1) x (meaning x is assigned in the true branch) or not (-1 x) (meaning x is assigned in the false branch)?

[alrz]

I think you want:

if (span.IndexOf(pattern) is not -1 and var x)
{
   Console.WriteLine(x);  
}