[Bug c++/47436] New: Variadic base-specifier-list of union rejected

[Bug c++/47436] New: Variadic base-specifier-list of union rejected

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

           Summary: Variadic base-specifier-list of union rejected
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: schaub.johannes@googlemail.com


GCC doesn't accept this

  template<typename ...T> union A : T... { };
  A<> a;

It complains at parse time of "A" about "error: derived union 'A<T>' invalid".
However it doesn't know at that time whether or not the union will have bases
yet when instantiated.

[Bug c++/47436] Variadic base-specifier-list of union rejected

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

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

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

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2011-08-22 22:26:07 UTC ---
Based on the FDIS wording (14.6 p. 8) this example should be ill-formed. But
maybe the diagnostic could be improved?

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Variadic                    |[C++0x] Variadic
                   |base-specifier-list of      |base-specifier-list of
                   |union rejected              |union rejected

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-09-22 18:02:45 UTC ---
Suggestions about a better error message? (should be easy to change)

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2011-09-22 18:11:24 UTC ---
(In reply to comment #2)
> Suggestions about a better error message? (should be easy to change)

What about:

"error: every valid template specialization requires an empty template
parameter pack"

? I don't know how much local information is available, if there is more
available, so following would be better:

"error: every valid specialization of template A requires an empty template
parameter pack T"

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[schaub.johannes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

--- Comment #4 from Johannes Schaub <schaub.johannes at googlemail dot com> 2011-09-22 19:01:51 UTC ---
(In reply to comment #3)
> (In reply to comment #2)
> > Suggestions about a better error message? (should be easy to change)
> 
> What about:
> 
> "error: every valid template specialization requires an empty template
> parameter pack"
> 
> ? I don't know how much local information is available, if there is more
> available, so following would be better:
> 
> "error: every valid specialization of template A requires an empty template
> parameter pack T"

Hmm, this is very technical though. When the user wrote that code, most
probably they either don't want an union, but a class/struct, or they don't
want to derive from something. 

For the tiny fraction of users that *do* want to write that exact code, they
most probably expect it to be valid because they most probably think that if T
is empty, it will expand to nothing. 

With the proposed wording of the diagnostics, in both cases the diagnostic is
suboptimal

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-09-22 19:19:21 UTC ---
I'd go for "error: variadic template A<T> is invalid because every valid
specialization requires an empty template parameter pack"

but I'm not sure that would be needed if the original error simply said "error:
unions cannot have base classes" - I think that's clear enough

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

Paolo Carlini <paolo.carlini at oracle dot com> changed:

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

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-09-22 20:48:59 UTC ---
I'm reading 14.6/8 and I tend to agree with Jon that not mentioning A<T> would
be probably less misleading. However, I'm afraid we have the recurring problem
that, missing caret diagnostics, we want to name something.

Let's see if Jason has an opinion, maybe for now we want to just live with the
current error message...

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> 2011-09-22 22:05:54 UTC ---
(In reply to comment #6)
> I'm reading 14.6/8 and I tend to agree with Jon that not mentioning A<T> would
> be probably less misleading. However, I'm afraid we have the recurring problem
> that, missing caret diagnostics, we want to name something.

You don't need to name something to give a source location, just use error_at.

> Let's see if Jason has an opinion, maybe for now we want to just live with the
> current error message...

I think the current message is clear enough.  Only people writing compiler
tests are ever going to hit it.

[Bug c++/47436] [C++0x] Variadic base-specifier-list of union rejected

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47436

Paolo Carlini <paolo.carlini at oracle dot com> changed:

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

--- Comment #8 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-09-22 22:46:26 UTC ---
Good, let's close this as invalid, then.

Thanks for clarifying the error_at thing, I should give it more thought: I was
under the impression that we tend to avoid (not a strict rule) things like
"error: unions cannot have base classes" because on a given line a lot can
happen (other errors together with good code) and an accurate column number is
anyway less convenient (for sure to a human like me!) than a name.