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The changes to Complex LGTM. I'm comfortable with us saying that the properties, Equals, and ToString methods will never mutate. It's probably even worth refactoring Complex.Sqrt so it can be a readonly struct.

BitIntegerCalculator.BitsBuffer on the other hand is an internal type and so I'm not sure if its worth actually updating it to be readonly.

@tannergooding Updated Complex (fix Sqrt method and mark whole struct)
Should I revert BitIntegerCalculator.BitsBuffer now or we are waiting somebody else's opinion?

Should I revert BitIntegerCalculator.BitsBuffer now

Yes, I think so.

In general, it is not very valuable to use readonly on types that are designed to be mutable, such as BitsBuffer.

@jkotas Done
Some unrelated errors in CI

This PR is being marked as a draft per #46675 (comment).

I think this one is good and we can merge it. Do you agree?

Yes, just need to do a final review pass over the Sqrt refactoring

This PR is being marked as a draft per #46675 (comment).

This is one we are wanting to take. Complex is a small numeric struct type where the entire struct should have been readonly.

Like many small numeric types, it is conceptually its own primitive and mutating one of its two instance fields isn't really desirable.

Hello @tannergooding!

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I've requeued the failing job and set auto-merge.