Comment 8 for bug 1914094

The atan case looks fixable, though ugly. There are several other cases where numeric-contagion is assumed to return a numeric-type, not a union type, though it explicitly says it can return a union type. I need to understand how and why the code chooses to represent types in certain ways to make progress which will take some time.

I added a patch to handle the decf case here https://github.com/ajberkley/sbcl/commit/2bec2009920685254aad0af1bc36551ac6399839

It is triggered also with the simpler:

(let ((x 813.0d0))
   (decf x))

The problem was maybe-infer-iteration-var-type which assumes that numeric-contagion always returns a numeric-type and not a union of numeric types. Similar to the atan case. If the comment on numeric-contagion is correct, then fixing this is good. I will need to think through whether we would want to do better than just failing in this case (assuming that in a later pass of the constraint phase the types will be tighter and we will succeed?).