DataFlow analysis requires both named and underlying ItemX fields be assigned

[VSadov]

The following should not be an error.

        static void Main(string[] args)
        {
            (string A, string B) ss;

            ss.A = "q";
            ss.Item2 = "w";

            System.Console.WriteLine(ss);
        }

Severity Code Description Project File Line Suppression State
Error CS0165 Use of unassigned local variable 'ss' Program.cs 18 Active

[VSadov]

Note that the bug exists in both C# and VB for the same reasons and the fix should be implemented for both languages.

One of possible results of bad def assignment analysis could be wrong async captures (since that relies on unassignendness at suspend points).
When fixing, Check that all is ok when tuple locals are captured.

[jcouv]

Fixed by #13107