[Bug c++/101500] New: gcc accepts the code with extra curly braces

[Bug c++/101500] New: gcc accepts the code with extra curly braces

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

            Bug ID: 101500
           Summary: gcc accepts the code with extra curly braces
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The following ill-formed code is the illustration
```
struct A
{
    A() {}
    A( A&& ) = delete;
    ~A() {}
};

int main() {
    const A a = {{A{}}}; //error line
}
```
https://gcc.godbolt.org/z/eq5WhTfz3

[Bug c++/101500] [9/10/11/12 Regression] gcc accepts the code with extra curly braces

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |10.3.0, 11.1.0, 12.0,
                   |                            |8.1.0, 9.4.0
                 CC|                            |jason at gcc dot gnu.org
      Known to work|                            |7.5.0
   Last reconfirmed|                            |2021-07-19
     Ever confirmed|0                           |1
            Summary|gcc accepts the code with   |[9/10/11/12 Regression] gcc
                   |extra curly braces          |accepts the code with extra
                   |                            |curly braces
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
EDG v6.2 also accepts this, but Clang and MSVC don't.

GCC started to accept it at:

    PR c++/85092 - C++17 ICE with unused list constructor.

            * call.c (conv_binds_ref_to_prvalue): Also count ck_identity
            from a TARGET_EXPR.

    From-SVN: r259052


I'll confirm it as a regression, since it doesn't look like that intended to
change this case.

[Bug c++/101500] [9/10/11/12 Regression] gcc accepts the code with extra curly braces

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

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

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

[Bug c++/101500] [9/10/11/12 Regression] gcc accepts the code with extra curly braces

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/101500] [C++17] [DR2311] gcc accepts the code with extra curly braces

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11/12 Regression] gcc |[C++17] [DR2311] gcc
                   |accepts the code with extra |accepts the code with extra
                   |curly braces                |curly braces
             Status|NEW                         |SUSPENDED
                URL|                            |wg21.link/cwg2311

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
This seems like an instance of issue 2311: with C++17 mandatory copy elision,
do we really want an actual call to the move constructor in this case?  The
testcase is rejected in C++14 mode, as expected.

The standard says that we try to call a constructor, initializing the argument
with {A{}}.  So we in turn look for an A constructor to initialize the
argument, initializing its argument from A{}.  Under 2311, does this result in
still an rvalue?  And the enclosing constructor call, still the same rvalue?

This seems parallel to

const A a = A(A(A()));

which is also accepted by clang in C++17 mode.

Perhaps we want to raise the priority of issue 2311.

[Bug c++/101500] [C++17] [DR2311] gcc accepts the code with extra curly braces

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101500

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

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