[Bug c++/115676] New: gcc allows invalid calling to implicitly-deleted default constructor of a template derived class in template function

[Bug c++/115676] New: gcc allows invalid calling to implicitly-deleted default constructor of a template derived class in template function

[rush102333 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115676

            Bug ID: 115676
           Summary: gcc allows invalid calling to implicitly-deleted
                    default constructor of a template derived class in
                    template function
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rush102333 at gmail dot com
  Target Milestone: ---

The following invalid code is accepted by gcc-13.2.0 and gcc-trunk:

~~~~~~~~~~~~~~~~~~~~~~
struct empty{

};
template <int size>
struct A
: public empty
{
    const int is_valid;
};

template <class T>
void check_a()
{
  const A<0> a {};
}
~~~~~~~~~~~~~~~~~~~~~~

It can be seen that the template class "A" is non-trivial because it has an
empty base class. Thus, the default constructor of "A" should be implicitly
deleted, and the initialization statement "const A<0> a{}" should be invalid. 

But when the initialization happens in a template function, GCC seems to ignore
the error. Changing "check_a" to a non-template function can make the compiler
reject the code by giving the following error message:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>: In function 'void check_a()':
<source>:14:17: error: use of deleted function 'A<0>::A()'
   14 |   const A<0> a {};
      |                 ^
<source>:5:8: note: 'A<0>::A()' is implicitly deleted because the default
definition would be ill-formed:
    5 | struct A
      |        ^
<source>: At global scope:
<source>:5:8: error: uninitialized const member in 'struct A<0>'
<source>:8:15: note: 'const int A<0>::is_valid' should be initialized
    8 |     const int is_valid;
      |               ^~~~~~~~
<source>: In function 'void check_a()':
<source>:14:17: note: use '-fdiagnostics-all-candidates' to display considered
candidates
   14 |   const A<0> a {};

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Please check https://godbolt.org/z/nGxPdn9jf

[Bug c++/115676] [c++11-C++14] gcc allows invalid calling to implicitly-deleted default constructor of a template derived class (non-depdenent) in template function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115676

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|gcc allows invalid calling  |[c++11-C++14] gcc allows
                   |to implicitly-deleted       |invalid calling to
                   |default constructor of a    |implicitly-deleted default
                   |template derived class in   |constructor of a template
                   |template function           |derived class
                   |                            |(non-depdenent) in template
                   |                            |function
                 CC|                            |pinskia at gcc dot gnu.org

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Actually I think this is just QoI case. IIRC the compiler does not require
error out if inside a template only if it is instantiated.

[Bug c++/115676] [c++11-C++14] gcc allows invalid calling to implicitly-deleted default constructor of a template derived class (non-dependent) in template function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115676

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Yes see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55361#c5 for that ...

[Bug c++/115676] [c++11-C++14] gcc allows invalid calling to implicitly-deleted default constructor of a template derived class (non-dependent) in template function

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115676

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #3 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Clang and MSVC give no diagnostic as well.

So like PR55361, "unless this is recategorised as a diagnostic enhancement
request this is clearly invalid, the compiler is not required to give an error
for this code."