[Bug c++/104142] New: [9/10/11/12] Spurios warning unused-variable

[Bug c++/104142] New: [9/10/11/12] Spurios warning unused-variable

[phdiv at fastmail dot fm]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

            Bug ID: 104142
           Summary: [9/10/11/12] Spurios warning unused-variable
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: phdiv at fastmail dot fm
  Target Milestone: ---

Created attachment 52238
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52238&action=edit
Reproducer.

> cat c.cpp
struct A { };
static const A a;

struct B { B()=default; };
static const B b;


> g++ -Wunused-variable -Wunused-const-variable -c c.cpp
c.cpp:5:16: warning: 'b' defined but not used [-Wunused-variable]
    5 | static const B b;
      |                ^
c.cpp:2:16: warning: 'a' defined but not used [-Wunused-const-variable=]
    2 | static const A a;
      |                ^


I would expect the warning unused-const-variable to be issued for both a and b.

[Bug c++/104142] Spurios warning unused-variable

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11/12] Spurios        |Spurios warning
                   |warning unused-variable     |unused-variable
           Keywords|                            |diagnostic
   Last reconfirmed|                            |2022-01-20
             Status|UNCONFIRMED                 |NEW
      Known to fail|                            |10.3.0, 11.2.0, 12.0, 9.4.0
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
'b' is likely not marked readonly as it is initialized.

[Bug c++/104142] [9/10/11/12 Regression] Spurios warning unused-variable on const static variable and defaulted constructor

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |9.5
            Summary|Spurios warning             |[9/10/11/12 Regression]
                   |unused-variable on const    |Spurios warning
                   |static variable and         |unused-variable on const
                   |defaulted constructor       |static variable and
                   |                            |defaulted constructor

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There was no -Wunused-variable warning for b before GCC 8.
-Wunused-const-variable didn't warn about b either in GCC 8 (and before).

[Bug c++/104142] [9/10/11/12 Regression] Spurios warning unused-variable on const static variable and defaulted constructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

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

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It isn't clear what exactly is the regression.
We warn on b with -Wunused-variable since
r9-412-gb46b715d5b838d9869f89d3594ebf7d0b7cb374c
A and B aren't the same, one has explicit defaulted ctor, the other has
implicit ctor.
-Wunused-const-variable warns about a and not b since its introduction in
r6-3283-g0f876f22887f239cedca64ff170b71e5f1daaf54

[Bug c++/104142] [9/10/11/12 Regression] Spurios warning unused-variable on const static variable and defaulted constructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
If we want the same warning, we'd need to probably special case const vars with
defaulted default ctors and empty classes, if it isn't empty class, both of
these are rejected already:
pr104142.C:2:16: error: uninitialized ‘const a’ [-fpermissive]
    2 | static const A a;
      |                ^
pr104142.C:1:8: note: ‘const struct A’ has no user-provided default constructor
    1 | struct A { int a; };
      |        ^
pr104142.C:1:16: note: and the implicitly-defined constructor does not
initialize ‘int A::a’
    1 | struct A { int a; };
      |                ^
pr104142.C:5:16: error: uninitialized ‘const b’ [-fpermissive]
    5 | static const B b;
      |                ^
pr104142.C:4:8: note: ‘const struct B’ has no user-provided default constructor
    4 | struct B { B()=default; int b; };
      |        ^
pr104142.C:4:12: note: constructor is not user-provided because it is
explicitly defaulted in the class body
    4 | struct B { B()=default; int b; };
      |            ^
pr104142.C:4:29: note: and the implicitly-defined constructor does not
initialize ‘int B::b’
    4 | struct B { B()=default; int b; };
      |                             ^

[Bug c++/104142] [9/10/11/12 Regression] Spurios warning unused-variable on const static variable and defaulted constructor

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

Jason Merrill <jason at gcc dot gnu.org> changed:

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

[Bug c++/104142] [9/10/11/12 Regression] Spurios warning unused-variable on const static variable and defaulted constructor

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:0899b22d9329c1e8fc12d6f0deee0f87dcc0163a

commit r12-8099-g0899b22d9329c1e8fc12d6f0deee0f87dcc0163a
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 11 14:50:14 2022 -0400

    c++: rodata and defaulted ctor [PR104142]

    Trivial initialization shouldn't bump a variable out of .rodata; if the
    result of build_aggr_init is an empty STATEMENT_LIST, throw it away.

            PR c++/104142

    gcc/cp/ChangeLog:

            * decl.cc (check_initializer): Check TREE_SIDE_EFFECTS.

    gcc/testsuite/ChangeLog:

            * g++.dg/opt/const7.C: New test.

[Bug c++/104142] [9/10/11 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[mh+gcc at glandium dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

Mike Hommey <mh+gcc at glandium dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mh+gcc at glandium dot org

--- Comment #6 from Mike Hommey <mh+gcc at glandium dot org> ---
There's a different warning that interestingly only happens on 11.2 and 11.3
(not 11.1 or earlier versions), that is fixed by the patch in this bug:

```
struct Foo {
  explicit constexpr Foo(char) {}
};

const Foo foo[] = {};
```

```
<source>: In function 'void __static_initialization_and_destruction_0(int,
int)':
<source>:5:11: error: statement has no effect [-Werror=unused-value]
    5 | const Foo foo[] = {};
      |           ^~~
```

[Bug c++/104142] [9/10/11 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[mh+gcc at glandium dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

--- Comment #7 from Mike Hommey <mh+gcc at glandium dot org> ---
(In reply to Mike Hommey from comment #6)
> There's a different warning that interestingly only happens on 11.2 and 11.3
> (not 11.1 or earlier versions), that is fixed by the patch in this bug:

Forgot to mention: that warning was a regression from bug 101029

[Bug c++/104142] [9/10/11 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:5296b77556da4682d5a1e2318c0643affaa00563

commit r11-9981-g5296b77556da4682d5a1e2318c0643affaa00563
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 11 14:50:14 2022 -0400

    c++: rodata and defaulted ctor [PR104142]

    Trivial initialization shouldn't bump a variable out of .rodata; if the
    result of build_aggr_init is an empty STATEMENT_LIST, throw it away.

            PR c++/104142

    gcc/cp/ChangeLog:

            * decl.c (check_initializer): Check TREE_SIDE_EFFECTS.

    gcc/testsuite/ChangeLog:

            * g++.dg/opt/const7.C: New test.

[Bug c++/104142] [9/10 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:cfa3a04be981f63af41f9a960df06e84c2634c5f

commit r10-10729-gcfa3a04be981f63af41f9a960df06e84c2634c5f
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Apr 11 14:50:14 2022 -0400

    c++: rodata and defaulted ctor [PR104142]

    Trivial initialization shouldn't bump a variable out of .rodata; if the
    result of build_aggr_init is an empty STATEMENT_LIST, throw it away.

            PR c++/104142

    gcc/cp/ChangeLog:

            * decl.c (check_initializer): Check TREE_SIDE_EFFECTS.

    gcc/testsuite/ChangeLog:

            * g++.dg/opt/const7.C: New test.

[Bug c++/104142] [10 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |10.4

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 branch is being closed

[Bug c++/104142] [10 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug c++/104142] [10 Regression] Spurious warning unused-variable on const static variable and defaulted constructor

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104142

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|10.5                        |10.4
      Known to work|                            |10.4.0
             Status|ASSIGNED                    |RESOLVED

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed in 10.4 already.