[Patch, libstdc++/65420] Use constexpr variables as regex_constans flags

[Daniel Krügler]

2015-03-15 9:09 GMT+01:00 Tim Shen <timshen91@gmail.com>:
> Did a little bit refectoring on regex_constants, so that users won't
> be polluted too much (they are not enum types anymore).

Your implementation choice is an interesting approach. I believe that
a strict reading of the library specification does not allow that
form, because you are using as bitmask type a class type that is not
std::bitset:

[bitmask.types]:
Each bitmask type can be implemented as an enumerated type that
overloads certain operators, as an integer type, or as a bitset
(20.6).

One could argue that this restriction to this specific set of types
could be considered as a Standard Library defect, but no-one has yet
submitted one ;-), so it depends on the decision of the maintainers
here whether it would be better to follow that strict reading of the
Standard or not.

What I would consider as more controversial are the following things:

a) The class takes advantage of a deprecated rule to implicitly
declare a copy-assignment operator (because it has a user-declared
copy-constructor). I suggest to fix that and add a defaulted one
before at some future point this bitmask type is no longer
copy-assignable.

b) Basically no operation is marked as noexcept. This is
user-observable and seems like an unfortunate outcome.

c) The presence of a non-explicit conversion to bool allows users to
unintentionally write code like

std::regex_constants::syntax_option_type a = ..;
[..]
int x = a + 3; // Oops

The presence of this function also *seemingly* allows to implicitly
convert syntax_option_type to any integer type:

int x = a; // Oops

I suggest to make this conversion function explicit.

d) One could consider to mark the mutable operations
(syntax_option_type& operator&=(syntax_option_type)) as constexpr in
C++14 mode, but that is just an idea, not a required functionality.

- Daniel