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

[Nicolas Lesser via Phabricator via cfe-commits]

Rakete1111 added inline comments.


================
Comment at: clang/lib/Parse/ParseDecl.cpp:4321
+             isCXXDeclarationSpecifier(ITC_Never, TPResult::True) !=
+                 TPResult::True) ||
+            (!getLangOpts().CPlusPlus && !isDeclarationSpecifier(ITC_Yes))) {
----------------
rsmith wrote:
> It seems like a wording oversight that we don't assume `typename` in an //enum-base//. Probably would be good to raise this on the core reflector.
Do you know why this isn't allowed in `operator` ids?


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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:
> 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.


================
Comment at: clang/lib/Sema/SemaTemplate.cpp:3377-3379
       // FIXME: This is not quite correct recovery as we don't transform SS
       // into the corresponding dependent form (and we don't diagnose missing
       // 'template' keywords within SS as a result).
----------------
rsmith wrote:
> This FIXME is concerning. Is this a problem with this patch? (Is the FIXME wrong now?)
Yes you're right, I believe it not longer applies due to the change in ParseDecl.cpp in ParseDeclarationSpecifiers.


Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D53847