Definite assignment should be informed by nullable checking.

[otac0n]

Definite assignment should be informed by nullable checking. For example:

            bool symbolicA, symbolicB;
            if ((symbolicA = aObject is KnownConstant symbolA) && (symbolicB = bObject is KnownConstant symbolB))
            {
                return string.Compare(symbolB.Name, symbolA.Name, StringComparison.Ordinal);
            }

or

            var symbolicA = aObject is KnownConstant symbolA;
            var symbolicB = bObject is KnownConstant symbolB;
            if (symbolicA && symbolicB)
            {
                return string.Compare(symbolB.Name, symbolA.Name, StringComparison.Ordinal);
            }

These could feasibly be relaxed to treat symbolA and symbolB as definitely assigned.

As it stands, we get this slightly confusing set of statements from the language tools:

'symbolA' is not null here
Use of unassigned local variable 'symbolA'

Compare this to the very similar:

            if (aObject is KnownConstant symbolA && bObject is KnownConstant symbolB)
            {
                return string.Compare(symbolB.Name, symbolA.Name, StringComparison.Ordinal);
            }

Which is currently allowed.

[HaloFour]

Similar: #2388

The compiler and language has no concept of tracking that kind of flow analysis through intermediary values. I could see that getting very complicated very quickly.