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
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D53847/new/
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