I don't think of DAYOFMONTH() as a type-conversion. I think of it as a one-way data transformation. I'm extracting data, not converting the type. This may sound like semantics, but it's not. For example, do you consider accessors as type converters? You can think of DAYOFMONTH as an accessor on a DATE column. Now, if you were to write a DAYOFMONTH function in Oracle, that wouldn't be a type converter. It may utilize type-conversion as part of its process, but that's irrelevant to the user.

Optimizations aren't wrong, and they don't violate the hard rule I mentioned. Well, they do, but they do so because they're increasing developer cost to reduce some other cost that's been deemed more important. The hard rule I mentioned assumes you want to minimize developer cost. If you don't, then violate the rule. :-)

Goto has plenty of uses. Unfettered goto is less useful. Remember - next and last are both goto statements with severe restrictions. The only hard and fast rule I can think of is that every rule has an exception, usually because you sidestepped one of its assumptions.