[Bug c++/96179] New: g++-10.1 silently doesn't push_back the return of a void function

[Bug c++/96179] New: g++-10.1 silently doesn't push_back the return of a void function

[agadethrowaway at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

            Bug ID: 96179
           Summary: g++-10.1 silently doesn't push_back the return of a
                    void function
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: agadethrowaway at gmail dot com
  Target Milestone: ---

Created attachment 48866
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48866&action=edit
Minimal reproducing example

#include <vector>
using namespace std;

struct dummy{
        int a;
};

void Modify_Dummy(dummy &d){
        d.a=1;
}

template <bool bla=true> void Templated_Function(){
        vector<dummy> A;
        A.push_back(Modify_Dummy(dummy{0}));
}

int main(){
        Templated_Function();
}

Compiled with

all:
        g++-10 Void_Pushback.cpp -o Void_Pushback

using:

g++-10 (Ubuntu 10-20200411-0ubuntu1) 10.0.1 20200411 (experimental) [master
revision bb87d5cc77d:75961caccb7:f883c46b4877f637e0fa5025b4d6b5c9040ec566]

Copy of code attached.

Code should not compile. The push_back instruction is silently ignored and the
vector stays empty.

[Bug c++/96179] [10/11 Regression] g++-10.1 silently doesn't push_back the return of a void function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.2
                 CC|                            |mpolacek at gcc dot gnu.org
            Summary|g++-10.1 silently doesn't   |[10/11 Regression] g++-10.1
                   |push_back the return of a   |silently doesn't push_back
                   |void function               |the return of a void
                   |                            |function
     Ever confirmed|0                           |1
      Known to work|                            |9.3.1
           Keywords|                            |accepts-invalid
      Known to fail|                            |10.1.1, 11.0
   Last reconfirmed|                            |2020-07-13
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Confirmed. This started with r10-7096 (like PR 95789).

A reduced example shows that GCC even warns the vector variable is unused:

template<typename T> struct vector
{
  void push_back(T) { }
};

struct dummy{
        int a;
};

void Modify_Dummy(dummy &d){
        d.a=1;
}

template <bool bla=true> void Templated_Function(){
        vector<dummy> A;
        A.push_back(Modify_Dummy(dummy{0}));
}

int main(){
        Templated_Function();
}


96179.C: In instantiation of 'void Templated_Function() [with bool bla =
true]':
96179.C:20:28:   required from here
96179.C:15:23: warning: variable 'A' set but not used
[-Wunused-but-set-variable]
   15 |         vector<dummy> A;
      |                       ^

[Bug c++/96179] [10/11 Regression] g++-10.1 silently doesn't push_back the return of a void function

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org
           Priority|P3                          |P2

[Bug c++/96179] [10/11 Regression] g++-10.1 silently doesn't push_back the return of a void function

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/96179] [10/11 Regression] g++-10.1 silently doesn't push_back the return of a void function

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This is also fixed by the patch for PR 95789.  But another test is useful.

*** This bug has been marked as a duplicate of bug 95789 ***

[Bug c++/96179] [10/11 Regression] g++-10.1 silently doesn't push_back the return of a void function

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

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

https://gcc.gnu.org/g:8e64d182850560dbedfabb88aac90d4fc6155067

commit r11-2097-g8e64d182850560dbedfabb88aac90d4fc6155067
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Jun 22 21:26:49 2020 -0400

    c++: Make convert_like complain about bad ck_ref_bind again [PR95789]

    convert_like issues errors about bad_p conversions at the beginning
    of the function, but in the ck_ref_bind case, it only issues them
    after we've called convert_like on the next conversion.

    This doesn't work as expected since r10-7096 because when we see
    a conversion from/to class type in a template, we return early, thereby
    missing the error, and a bad_p conversion goes by undetected.  That
    made the attached test to compile even though it should not.

    I had thought that I could just move the ck_ref_bind/bad_p errors
    above to the rest of them, but that regressed diagnostics because
    expr then wasn't converted yet by the nested convert_like_real call.

    So, for bad_p conversions, do the normal processing, but still return
    the IMPLICIT_CONV_EXPR to avoid introducing trees that the template
    processing can't handle well.  This I achieved by adding a wrapper
    function.

    gcc/cp/ChangeLog:

            PR c++/95789
            PR c++/96104
            PR c++/96179
            * call.c (convert_like_real_1): Renamed from convert_like_real.
            (convert_like_real): New wrapper for convert_like_real_1.

    gcc/testsuite/ChangeLog:

            PR c++/95789
            PR c++/96104
            PR c++/96179
            * g++.dg/conversion/ref4.C: New test.
            * g++.dg/conversion/ref5.C: New test.
            * g++.dg/conversion/ref6.C: New test.

[Bug c++/96179] [10/11 Regression] g++-10.1 silently doesn't push_back the return of a void function

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96179

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:315b87f63bcde4c881e9963c39d57258da08ccb5

commit r10-8497-g315b87f63bcde4c881e9963c39d57258da08ccb5
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Jun 22 21:26:49 2020 -0400

    c++: Make convert_like complain about bad ck_ref_bind again [PR95789]

    convert_like issues errors about bad_p conversions at the beginning
    of the function, but in the ck_ref_bind case, it only issues them
    after we've called convert_like on the next conversion.

    This doesn't work as expected since r10-7096 because when we see
    a conversion from/to class type in a template, we return early, thereby
    missing the error, and a bad_p conversion goes by undetected.  That
    made the attached test to compile even though it should not.

    I had thought that I could just move the ck_ref_bind/bad_p errors
    above to the rest of them, but that regressed diagnostics because
    expr then wasn't converted yet by the nested convert_like_real call.

    So, for bad_p conversions, do the normal processing, but still return
    the IMPLICIT_CONV_EXPR to avoid introducing trees that the template
    processing can't handle well.  This I achieved by adding a wrapper
    function.

    gcc/cp/ChangeLog:

            PR c++/95789
            PR c++/96104
            PR c++/96179
            * call.c (convert_like_real_1): Renamed from convert_like_real.
            (convert_like_real): New wrapper for convert_like_real_1.

    gcc/testsuite/ChangeLog:

            PR c++/95789
            PR c++/96104
            PR c++/96179
            * g++.dg/conversion/ref4.C: New test.
            * g++.dg/conversion/ref5.C: New test.
            * g++.dg/conversion/ref6.C: New test.

    (cherry picked from commit 8e64d182850560dbedfabb88aac90d4fc6155067)