Allow use of increment ++ and decrement -- operators with null conditional assignments

[fitdev]

Recently C# introduced support for null conditional assignments:

foo?.num = 1;

However it seems that while compound assignment operators were included, increment and decrement operators were not, such that:

foo?.num += 1;

is valid, but:

foo?.num++;

is an error.

It would be nice if the same logic also applies to ++ and -- operators.

[RikkiGibson]

For context: https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-10-28.md#increment-and-decrement-operators-in-null-conditional-access

We decided to disallow these operators. If folks find scenarios where they need these operators, we might revisit this decision in the future.

[CyrusNajmabadi]

Yup. We're not opposed to them per-se. But it's more impl/spec cost. And there's an easy workaround.

[fitdev]

Hopefully this will be implemented at some point if anything then for completeness sake. Code looks a bit awkward without it and some analyzers also complain: "use ++ instead of +=1".

[RikkiGibson]

It feels reasonable enough to support the postfix case only, and, if you try to use them in prefix form ++a?.b, you would get a similar error message as you get today. I think when I first wrote the question for LDM, I was uncomfortable with that mismatch between prefix and postfix support, but also uncomfortable with the increased engineering effort that would be required to support the prefix form.