[PATCH] D53847: [C++2a] P0634r3: Down with typename!

[Erich Keane via Phabricator via cfe-commits]

erichkeane added inline comments.


================
Comment at: clang/lib/Parse/ParseDecl.cpp:5592
+bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
+                                     bool IsFriend) {
   TentativeParsingAction TPA(*this);
----------------
aaron.ballman wrote:
> shafik wrote:
> > Instead of adding yet another `bool` flag maybe we can consider using something like `enum isFriend : bool {No, Yes}`.
> > 
> > I am sure @aaron.ballman will want to chime in here as well.
> Heh, so this is where I get worried about the scalability of using enums for these. We really want to use three different enums here, but do we really want to *add* three different enums? I'm unconvinced.
> 
> However, if we can come up with some template magic to allow for named bool parameters as a generic interface, that would be valuable to use.
I prefer enums over bools TBH, even if we end up with a million of then somewhere.

That said, what about:

https://godbolt.org/z/Kz6jdjobj

```
template<typename SpecificThing>
class Is {
    Is(bool v) : value(v){}
  public:
  bool value;
  static const Is Yes() { return Is{true};}
  static const Is No() { return Is{false};}

  operator bool() { return value; }
};

class Friend{}; // #1
 
void foo(Is<Friend> f) {
    if (f) {
        ///...
    }
}

void baz() {
    foo(Is<Friend>::Yes());
}
```

Adding a 'new' thing is as simple as just adding #1 for anything we care about.  We might want to put them in a namespace of some sort, but perhaps not awful?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D53847/new/

https://reviews.llvm.org/D53847