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

[Alan Zhao via Phabricator via cfe-commits]

ayzhao marked an inline comment as done.
ayzhao added inline comments.


================
Comment at: clang/lib/Parse/ParseDecl.cpp:5100-5101
   bool IsConstructor = false;
-  if (isDeclarationSpecifier())
+  if (isDeclarationSpecifier(ITC_Never))
     IsConstructor = true;
   else if (Tok.is(tok::identifier) ||
----------------
rsmith wrote:
> Rakete1111 wrote:
> > rsmith wrote:
> > > Oh, this could be a problem.
> > > 
> > > If this *is* a constructor declaration, then this is implicit typename context: this is either a "//parameter-declaration// in a //member-declaration//" ([temp.res]/5.2.3) or a "//parameter-declaration// in a //declarator// of a function or function template declaration whose //declarator-id// is qualified". But if it's *not* a constructor declaration, then this is either the //declarator-id// of a declaration or the //nested-name-specifier// of a pointer-to-member declarator:
> > > 
> > > ```
> > > template<typename T>
> > > struct C {
> > >   C(T::type); // implicit typename context
> > >   friend C (T::fn)(); // not implicit typename context, declarator-id of friend declaration
> > >   C(T::type::*x)[3]; // not implicit typename context, pointer-to-member type
> > > };
> > > ```
> > > 
> > > I think we need to use `ITC_Yes` here, in order to correctly disambiguate the first example above. Please add tests for the other two cases to make sure this doesn't break them -- but I'm worried this **will** break the second case, because it will incorrectly annotate `T::fn` as a type.
> > Yeah it does the break the second. Would just checking whether a `friend` is there be good enough? clang doesn't seem to actively propose to add  a friend if there's one missing, so if we add a `IsFriend` parameter and then check if it is set than always return `false`, it should work. Is that acceptable? It would break the nice error message you get if you write `friend C(int);`, but if we restrict it when `isDeclarationSpecifier` return false (with `Never`) would that be better (that's what I'm doing right now)?
> > 
> > Another solution would be to tentatively parse the whole thing, but that can be pretty expensive I believe.
> This seems a little fragile against future grammar changes, but I think the `IsFriend` check is correct -- I *think* the only way we can see a //qualified-id// here in a valid non-constructor, non-nested-name-specifier case is in a friend declaration.
I _think_ the first comment in this chain can be marked as done given that the test cases are now in `p5.cpp` and Clang compiles them without errors with the help of the `IsFriend` check.


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

https://reviews.llvm.org/D53847