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

[Alan Zhao via Phabricator via cfe-commits]

ayzhao added inline comments.


================
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).
----------------
Rakete1111 wrote:
> rsmith wrote:
> > Rakete1111 wrote:
> > > Rakete1111 wrote:
> > > > 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.
> > > Although I'm not that sure I fully understand what that comments alludes to.
> > The example would be something like this:
> > 
> > ```
> > template<typename T> struct X {
> >   template<typename U> struct Y { Y(); using V = int; };
> >   template<typename U> typename Y<U>::V f();
> > };
> > template<typename T> template<typename U>
> > X<T>::Y<U>::V X<T>::f() {} 
> > ```
> > 
> > Clang trunk today points out that the `typename` keyword is missing on the final line, but fails to point out that the `template` keyword is also missing. The reason is that in the case where that construct is valid:
> > 
> > ```
> > template<typename T> template<typename U>
> > X<T>::Y<U>::Y() {}
> > ```
> > 
> > ... we are "entering the context" of the //nested-name-specifier//, which means we don't need a `template` keyword.
> > 
> > If the FIXME is fixed, then we should diagnose the missing `template` in the above program.
> > 
> > Also, because we don't rebuild the //nested-name-specifier// as a dependent nested name specifier in this case, we fail to match it against the declaration in the class, so in my example above, we also produce an "out-of-line definition does not match" error.
> > 
> > 
> > A closely-related issue can be seen in an example such as:
> > 
> > ```
> > template<typename T> struct X {
> >   template<typename U> struct Y { Y(); typedef void V; }; 
> >   template<typename U> typename Y<U>::V::W f();
> > };
> > template<typename T> template<typename U>
> > X<T>::template Y<U>::V::W X<T>::f() { return 0; } 
> > ```
> > 
> > Here, we produce a completely bogus error:
> > 
> > ```
> > <stdin>:6:13: error: 'X::Y::V' (aka 'void') is not a class, namespace, or enumeration
> > X<T>::template Y<U>::V::W X<T>::f() { return 0; } 
> >             ^
> > ```
> > 
> > ... because we parse this in "entering context" mode and so resolve `X<T>::Y<U>::V` to the type in the primary template (that is, `void`). That's wrong: we should defer resolving this name to a type until we know what `T` and `U` are, or until we know that we're *actually* entering the context. Specifically, the above program can be extended as follows:
> > 
> > ```
> > template<> template<> struct X<int>::Y<int> {
> >   struct V { using W = int; };
> > };
> > void call(X<int> x) { x.f<int>(); } // OK, f returns int
> > ```
> > 
> > The above program should be accepted by this patch, if the FIXME is indeed now fixed. Please add it as a testcase :)
> Oh ok, got it thanks. So no, the program is still not accepted in clang, and a pretty dumb side effect of it is that 
> 
> ```
> template<typename T> struct X {
>   template<typename U> struct Y { Y(); using V = int; };
>   template<typename U> typename Y<U>::V f();
> };
> template<typename T> template<typename U>
> X<T>::Y<U>::V X<T>::f() {}
> ```
> 
> is accepted without the diagnostic for the missing `template`. :(
Currently looking into this.

It is interesting to note that for the following example in one of the parent comments:

```
template<typename T> struct X {
  template<typename U> struct Y { Y(); typedef void V; }; 
  template<typename U> typename Y<U>::V::W f();
};
template<typename T> template<typename U>
X<T>::template Y<U>::V::W X<T>::f() { return 0; }
```

GCC doesn't like the fact that the `template` keyword is on the final line if compiled with `-pedantic` [0]:

```
<source>:6:16: warning: keyword 'template' not allowed in declarator-id [-Wpedantic]
    6 | X<T>::template Y<U>::V::W X<T>::f() { return 0; }
      |                ^~~~
Compiler returned: 0
```

[0]: https://godbolt.org/z/dYddW3ErY


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