[Bug c++/51203] New: Recursive alias templates not working

[Bug c++/51203] New: Recursive alias templates not working

[pubby.8 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51203

             Bug #: 51203
           Summary: Recursive alias templates not working
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: pubby.8@gmail.com


-std=c++11
gcc version 4.7.0 20111112 (experimental) (GCC)

I presume this is correctly formed code:

template <typename t>
struct foo {
  template <int n>
  using next = typename foo<t>::next<n + 1>;
};

main.cpp:4:37: error: expected ‘;’ before ‘<’ token
main.cpp:4:37: error: expected unqualified-id before ‘<’ token

Interestingly, changing the line to this compiles so long as next isn't used:

  using next = typename foo<t>::next;

The error is very similar to this ill-formed code:

  using next<n> = int;

main.cpp:4:13: error: expected ‘=’ before ‘<’ token
main.cpp:4:13: error: expected type-specifier before ‘<’ token
main.cpp:4:13: error: expected ‘;’ before ‘<’ token
main.cpp:4:13: error: expected unqualified-id before ‘<’ token

[Bug c++/51203] Recursive alias templates not working

[pubby.8 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51203

--- Comment #1 from Pubby8 <pubby.8 at gmail dot com> 2011-11-18 07:53:36 UTC ---
Whoops! I seem to have left out the template qualifier.

template <typename t>
struct foo {
  template <int n>
  using next = typename foo<t>::next<n + 1>;
  template <>
  using next<10> = int; // not sure if alias templates can be specialized
};

int main() {
  foo<int>::next<1> x;
}

Although now I am getting a compiler seg fault:

main.cpp:5:13: error: explicit specialization in non-namespace scope ‘struct
foo<t>’
g++-4.7: internal compiler error: Segmentation fault (program cc1plus)

[Bug c++/51203] Recursive alias template specialization causes compiler segfault

[pubby.8 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51203

Pubby8 <pubby.8 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |trivial

[Bug c++/51203] [C++0x] Recursive alias template specialization causes compiler segfault

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51203

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #2 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2011-11-18 19:03:14 UTC ---
(In reply to comment #1)
> template <typename t>
> struct foo {
>   template <int n>
>   using next = typename foo<t>::next<n + 1>;
>   template <>
>   using next<10> = int; // not sure if alias templates can be specialized
> };

Alias templates cannot be specialized, this is by design.

[Bug c++/51203] [C++0x] Recursive alias template specialization causes compiler segfault

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51203

Dodji Seketeli <dodji at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |FIXED

--- Comment #3 from Dodji Seketeli <dodji at gcc dot gnu.org> 2011-11-25 12:59:05 UTC ---
commenting on the below,

    template <typename t>
    struct foo {
      template <int n>
      using next = typename foo<t>::next<n + 1>;//#1
      template <>
      using next<10> = int; // #2 not sure if alias templates can be
specialized
    };

I think #1 is invalid because [temp.alias]/3 says:

    "The type-id in an alias template declaration shall not refer to the
    alias template being declared."

In other words, the name "next" should not appear at the right hand
side of the "=" in the alias template declaration there.

And in #2, there is the additional invalid specialization of the alias
template, already raised by Daniel on comment #2.

Furthermore, I am not getting any ICE anymore on this from trunk of today
at revision r181718.

So I am closing this as fixed for now.

Thank you for filing this.