Default type for regular expressions

[e-kayrakli]

#14686 will add support for bytes regular expressions. In order to do that it makes regexp record generic on type exprType. The meaningful concrete types in this scenario are regexp(string) and regexp(bytes).

That PR does not assign a default value to exprType, but maybe it should. We have the following options:

1. No default type -- it'd be nice to have regexp neutral.
   This helps with some of the confusions I was having during the implementation of Add support for bytes regular expressions #14686. For example:

class Message {
  proc parse(r: regexp) { /* bla */ }
}

if regexp doesn't have default type, then, parse can be used with both string and bytes regular expressions. Otherwise, we'd be stuck with string only, or change the prototype to proc parse(r: regexp(?))

This is also a breaking change, but probably not very impactful. Note that you can use regular expressions without using this type explicitly:

var myRe = compile("regular expression string");

gives you a regexp(string). If the literal was bytes then it would give regexp(bytes)

2. default value is string
string should be the more common use for this class, and it might make sense to just make it default.

3. default value is bytes -- just for completeness. I don't think this is a real option

Even though #14686 chooses 1, I don't really have a preference between 1 and 2.

[BryantLam]

I would go with option 1: no default type. It's the least restrictive option between 1 and 2 if some person wants to make a custom string type for their use case.

[bradcray]

I think I said this elsewhere, but: I agree that it shouldn't have a default type. I could imagine it having a default type with deprecation warning for the 1.21 release to help people transition their existing codes that relied on it being string-only. I'm imagining this could be supported by having a 0-argument initializer that generated the warning and called a 1-argument initializer passing string in.