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

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/lib/Parse/ParseDecl.cpp:5592
+bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
+                                     bool IsFriend) {
   TentativeParsingAction TPA(*this);
----------------
erichkeane wrote:
> 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?
Yeah, this is along the lines of what I was thinking of! However, I'm still concerned about that approach because it involves adding a new type for every situation we have a bool. Empty classes to use as a tag definitely works, but I was hoping we could use a string literal rather than a tag type so that we don't have the extra compile time overhead of adding hundreds of new empty classes. e.g.,
```
void foo(Is<"Friend"> f) {
  if (f) {
     // ...
  }
}

void baz() {
  foo(Is<"Friend">::Yes); // Yay
  foo(Is<"Enemy">::Yes); // Error for type mismatch with Is<"Friend">
}
```
However, that might require compiling with C++20 (I don't recall), so it may not be a viable idea.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D53847/new/

https://reviews.llvm.org/D53847