[Bug c++/51786] New: [c++0x] Invalid declaration with decltype accepted

[Bug c++/51786] New: [c++0x] Invalid declaration with decltype accepted

[reichelt at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

             Bug #: 51786
           Summary: [c++0x] Invalid declaration with decltype accepted
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: reichelt@gcc.gnu.org


The error message "declaration does not declare anything" is not
triggered in certain cases when using decltype:

===================================
struct A {};
void foo() { decltype(A()); }
===================================

It *is* triggered if I write decltype(int) instead.

Another testcase is the following:

===================================
template<int> struct A
{
  A() { decltype(this); }
};

A<0> a;
===================================

It *is* triggered if I make A a non-template class.

This bug is similar to PR21120 where the same problems occured with "typeof".

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-01-16 18:10:46 UTC ---
I'm still not sure an error message should be triggered in the first place.

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com,
                   |                            |paolo.carlini at oracle dot
                   |                            |com

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-01-31 15:56:47 UTC ---
Hi Daniel, are you willing to help triaging this?

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-01-31 21:26:41 UTC ---
It seems to me that all these examples should be rejected (Thanks to Jens
Maurer for helping me here): At first it seems, that we can follow the grammar
chain starting from /declaration/ in 7 [dcl.dcl] via 

simple-declaration => decl-specifier-seq_opt init-declarator-list_opt; 

without the optional init-declarator-list reaching 

decl-specifier => type-specifier => trailing-type-specifier =>
simple-type-specifier => decltype(expression). 

But the show-stopper for this is 7 [dcl.dcl] p3:

"In a simple-declaration, the optional init-declarator-list can be omitted only
when declaring a class (Clause 9) or enumeration (7.2), that is, when the
decl-specifier-seq contains either a class-specifier, an elaborated-type-
specifier with a class-key (9.1), or an enum-specifier. [..]"

Neither of these three situations applies, so this is no valid
/simple-declaration/.

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-01-31
                 CC|paolo.carlini at oracle dot |
                   |com                         |
     Ever Confirmed|0                           |1

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-01-31 21:37:08 UTC ---
Ah, thanks a lot Daniel (and Jens). Let's confirm this.

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-16 10:49:22 UTC ---
The problem is that by the time at the end of cp_parser_simple_declaration we
call check_tag_decl (via shadow_tag), which is supposed to check that the
simple declaration is valid, we already called finish_decltype_type thus it
doesn't see the decltype, it sees something like:

void foo() { struct A; }

a valid simple declaration.

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |0coming.soon at gmail dot com

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> ---
*** Bug 57502 has been marked as a duplicate of this bug. ***

[Bug c++/51786] [c++0x] Invalid declaration with decltype accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51786

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |paolo.carlini at oracle dot com

--- Comment #7 from Paolo Carlini <paolo.carlini at oracle dot com> ---
On it.