Version Used:

master

Steps to Reproduce:

public class C
{
    public void M(object o)
    {
        if (o is string s { Length: 5 }) { }
    }
}

SharpLab.

Expected Behavior:

A more clear error message

Actual Behavior:

error CS1026: ) expected
error CS1002: ; expected
error CS1513: } expected

Alternative:

Allow the variable designation to come either before or after the property patterns.