[Bug c++/95328] New: structured binding of template type inside template function is reported as "incomplete class type"

[Bug c++/95328] New: structured binding of template type inside template function is reported as "incomplete class type"

[stefaan.deroeck at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

            Bug ID: 95328
           Summary: structured binding of template type inside template
                    function is reported as "incomplete class type"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefaan.deroeck at gmail dot com
  Target Milestone: ---

A structured binding assignment of a template type inside a template function
seems to be reported as "incomplete class type". 

Compiling below code apparently produces the warning (and thus failure with
-Werror) on all applicable GCC versions (gcc-7 segfaults, gcc-8 and up
including 11 reports the warning). Clang and intel compilers compile this
without warnings. 

This is the minimal code with which I can reproduce the issue. Removal of any
of the templating, or replacing the return type of makeData() by "auto", gets
rid of the warning. 

compilation flags: -std=c++17 -Werror

Reported error:
<source>: In function 'void func()':
<source>:14:8: error: structured binding refers to incomplete class type
'Data<int>' [-Werror]
   14 |   auto [a, b] = makeData<int>();
      |        ^~~~~~
cc1plus: all warnings being treated as errors



Code:
template <typename T>
struct Data
{
  int a, b;
};
template <typename T>
Data<T> makeData()
{
  return Data<T>();
}
template <class Anything>
void func()
{
  auto [a, b] = makeData<int>();
}
int main() { func<int>(); }

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[stefaan.deroeck at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

--- Comment #1 from Stefaan De Roeck <stefaan.deroeck at gmail dot com> ---
During further development, I nevertheless did find a shorter snippet to
reproduce the issue:

template <typename T>
struct Data
{
  int a, b;
};
template <class Anything>
void func()
{
  auto [a, b] = Data<int>();
}
int main() { func<int>(); }

If there were a way to disable this specific warning(/error), that would help
too...

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 48606
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48606&action=edit
gcc11-pr95328.patch

Untested fix.

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:3d8d5ddb539a5254c7ef83414377f4c74c7701d4

commit r11-706-g3d8d5ddb539a5254c7ef83414377f4c74c7701d4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu May 28 23:40:54 2020 +0200

    c++: Try to complete decomp types [PR95328]

    Two years ago Paolo has added the
      else if (processing_template_decl && !COMPLETE_TYPE_P (type))
        pedwarn (...);
    lines into cp_finish_decomp.  For type dependent decl we punt much earlier,
    but even for types which aren't type dependent COMPLETE_TYPE_P might be
    false as this testcase shows, so this patch tries to complete_type first
    (the reason for writing it that way is that it is then followed by another
    else if and if complete_type returns error_mark_node, we shouldn't report
    anything, as a bug should have been reported already.

    2020-05-28  Jakub Jelinek  <jakub@redhat.com>

            PR c++/95328
            * decl.c (cp_finish_decomp): Call complete_type before checking
            COMPLETE_TYPE_P.

            * g++.dg/cpp1z/decomp53.C: New test.

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2020-06-05
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed on the trunk so far.

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:808f30b0772733886cefe7dead50862c169bcb2b

commit r10-8295-g808f30b0772733886cefe7dead50862c169bcb2b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu May 28 23:40:54 2020 +0200

    c++: Try to complete decomp types [PR95328]

    Two years ago Paolo has added the
      else if (processing_template_decl && !COMPLETE_TYPE_P (type))
        pedwarn (...);
    lines into cp_finish_decomp.  For type dependent decl we punt much earlier,
    but even for types which aren't type dependent COMPLETE_TYPE_P might be
    false as this testcase shows, so this patch tries to complete_type first
    (the reason for writing it that way is that it is then followed by another
    else if and if complete_type returns error_mark_node, we shouldn't report
    anything, as a bug should have been reported already.

    2020-05-28  Jakub Jelinek  <jakub@redhat.com>

            PR c++/95328
            * decl.c (cp_finish_decomp): Call complete_type before checking
            COMPLETE_TYPE_P.

            * g++.dg/cpp1z/decomp53.C: New test.

    (cherry picked from commit 3d8d5ddb539a5254c7ef83414377f4c74c7701d4)

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:8efa945b308307fd9b0313705f5826a8499de405

commit r9-8900-g8efa945b308307fd9b0313705f5826a8499de405
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu May 28 23:40:54 2020 +0200

    c++: Try to complete decomp types [PR95328]

    Two years ago Paolo has added the
      else if (processing_template_decl && !COMPLETE_TYPE_P (type))
        pedwarn (...);
    lines into cp_finish_decomp.  For type dependent decl we punt much earlier,
    but even for types which aren't type dependent COMPLETE_TYPE_P might be
    false as this testcase shows, so this patch tries to complete_type first
    (the reason for writing it that way is that it is then followed by another
    else if and if complete_type returns error_mark_node, we shouldn't report
    anything, as a bug should have been reported already.

    2020-05-28  Jakub Jelinek  <jakub@redhat.com>

            PR c++/95328
            * decl.c (cp_finish_decomp): Call complete_type before checking
            COMPLETE_TYPE_P.

            * g++.dg/cpp1z/decomp53.C: New test.

    (cherry picked from commit 3d8d5ddb539a5254c7ef83414377f4c74c7701d4)

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed for 8.5 in r8-10506-g4163fd72a0606199c4ab867fde0efdff6346d6f3 and by the
above commit for 9.4+ too.

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

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

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

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 86930 has been marked as a duplicate of this bug. ***

[Bug c++/95328] structured binding of template type inside template function is reported as "incomplete class type"

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95328

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |10.2.0, 8.5.0, 9.4.0
      Known to fail|                            |10.1.0, 8.4.0, 9.3.0
   Target Milestone|---                         |8.5