[Bug c++/66350] typename should be forbidden in inhering constructors

[Bug c++/66350] typename should be forbidden in inhering constructors

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |accepts-invalid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-05-31
     Ever confirmed|0                           |1

[Bug c++/66350] typename should be forbidden in inhering constructors

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Here's a reduced test case compiled with -Wall in C++ 98 mode:

  $ cat t.cpp && ~/bin/gcc-5.1.0/bin/g++ -Wall -std=c++98 -c t.cpp
  template <class T> struct A { A (T) { } };
  template <class T> struct B: A<T> {
      using typename A<T>::A;
  };

  B<int> b (0);
  t.cpp:3:26: warning: inheriting constructors only available with -std=c++11
or -std=gnu++11
       using typename A<T>::A;
                          ^
I don't think gcc is incorrect in accepting the code either in C++ 98 mode as
an extension, or in C++ 11 mode.  In the latter mode, accepting it is in
keeping with the resolution of DR 147. Naming the constructor  (Clang accepts
the test case above in C++ 11 mode).  That is, when 'A<T>::A' nominates class
template A the name is instead taken to name the ctor of A.

[Bug c++/66350] typename should be forbidden in inhering constructors

[vgheorgh at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

--- Comment #2 from Vlad Gheorghiu <vgheorgh at gmail dot com> ---
(In reply to Martin Sebor from comment #1)
> Here's a reduced test case compiled with -Wall in C++ 98 mode:
> 
>   $ cat t.cpp && ~/bin/gcc-5.1.0/bin/g++ -Wall -std=c++98 -c t.cpp
>   template <class T> struct A { A (T) { } };
>   template <class T> struct B: A<T> {
>       using typename A<T>::A;
>   };
> 
>   B<int> b (0);
>   t.cpp:3:26: warning: inheriting constructors only available with
> -std=c++11 or -std=gnu++11
>        using typename A<T>::A;
>                           ^
> I don't think gcc is incorrect in accepting the code either in C++ 98 mode
> as an extension, or in C++ 11 mode.  In the latter mode, accepting it is in
> keeping with the resolution of DR 147. Naming the constructor  (Clang
> accepts the test case above in C++ 11 mode).  That is, when 'A<T>::A'
> nominates class template A the name is instead taken to name the ctor of A.

I tried compiling your test with Clang but it fails with the same error,
http://melpon.org/wandbox/permlink/7sGeB2XfPOHkOFfN

[Bug c++/66350] typename should be forbidden in inhering constructors

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
You're right.  My Clang test case wasn't quite the same.  Recent trunk does
fail to compile it.

[Bug c++/66350] typename should be forbidden in inhering constructors

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #4 from TC <rs2740 at gmail dot com> ---
(In reply to Martin Sebor from comment #1)

> I don't think gcc is incorrect in accepting the code either in C++ 98 mode
> as an extension, or in C++ 11 mode.  In the latter mode, accepting it is in
> keeping with the resolution of DR 147. Naming the constructor  (Clang
> accepts the test case above in C++ 11 mode).  That is, when 'A<T>::A'
> nominates class template A the name is instead taken to name the ctor of A.

The point is that since 'A<T>::A; names the constructor, it isn't a type, and
so using 'typename' shouldn't be allowed.

[Bug c++/66350] typename should be forbidden in inhering constructors

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Also, "using typename Base<T>::name;" would usually re-declare a type name in
the derived class' scope, but that's not what's happening here. It's
introducing inheriting constructors, not a type.

7.3.3 [namespace.udecl]/19 says:

  If a using-declaration uses the keyword typename and specifies a dependent
  name (14.6.2), the name introduced by the using-declaration is treated as a
  typedef-name (7.1.3).

and we definitely don't want it to be treated as a typedef-name, so "typename"
must not be used for an inheriting constructor.

[Bug c++/66350] typename should be forbidden in inheriting constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66350

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is just a dup of bug 34810.

*** This bug has been marked as a duplicate of bug 34810 ***