[Bug libstdc++/101427] New: std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[Bug libstdc++/101427] New: std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[federico.reghenzani at polimi dot it]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

            Bug ID: 101427
           Summary: std::get should refuse to compile if type is provided
                    and it is duplicated in std::tuple
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: federico.reghenzani at polimi dot it
  Target Milestone: ---

The following code should not compile:

----------------------------------------------------------
#include <tuple>

int main() {

        std::tuple<int, int> my_record;
        std::get<int>(my_record) = 10;

        return 0;
}
----------------------------------------------------------

Because `std::get<int>` accesses the `int` element which is duplicated. This is
not allowed by the standard: http://eel.is/c++draft/tuple#elem-5

Instead, it compiles and modifies only the first element in GCC 11.1.0 (while
it doesn't compile in 10.x.x). A full example here:
https://godbolt.org/z/YzorM9P8K

Expected behavior
-----------------
It should not compile and generate a compilation error.

Actual behavior
---------------
It compiles and (seems to) modify the first item of the tuple.


GCC Information
---------------
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.1.0 (GCC)

[Bug libstdc++/101427] [11/12 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |10.3.0
            Summary|std::get should refuse to   |[11/12 Regression] std::get
                   |compile if type is provided |should refuse to compile if
                   |and it is duplicated in     |type is provided and it is
                   |std::tuple                  |duplicated in std::tuple
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-07-12
   Target Milestone|---                         |11.2
      Known to fail|                            |11.1.0, 12.0
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

[Bug libstdc++/101427] [11/12 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The std::get<T> code stopped working as a result of r11-4693 which implemented
a core language change:

    c++: DR2303, ambiguous base deduction [PR97453]

    When there are two possible matches and one is a base of the other, choose
    the derived class rather than fail.

We should have had a libstdc++ test verifying the expected error, so that we'd
have noticed the library regression.

I will push the fix soon.

[Bug libstdc++/101427] [11/12 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:17855eed7fc76b2cee7fbbc26f84d3c8b99be13c

commit r12-2327-g17855eed7fc76b2cee7fbbc26f84d3c8b99be13c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 14 20:14:14 2021 +0100

    libstdc++: Fix std::get<T> for std::tuple [PR101427]

    The std::get<T> functions relied on deduction failing if more than one
    base class existed for the type T.  However the implementation of Core
    DR 2303 (in r11-4693) made deduction succeed (and select the
    more-derived base class).

    This rewrites the implementation of std::get<T> to explicitly check for
    more than one occurrence of T in the tuple elements, making it
    ill-formed again. Additionally, the large wall of overload resolution
    errors described in PR c++/101460 is avoided by making std::get<T> use
    __get_helper<I> directly instead of calling std::get<I>, and by adding a
    deleted overload of __get_helper<N> for out-of-range N.

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/101427
            * include/std/tuple (tuple_element): Improve static_assert text.
            (__get_helper): Add deleted overload.
            (get<i>(tuple<T...>&&), get<i>(const tuple<T...>&&)): Use
            __get_helper directly.
            (__get_helper2): Remove.
            (__find_uniq_type_in_pack): New constexpr helper function.
            (get<T>): Use __find_uniq_type_in_pack and __get_helper instead
            of __get_helper2.
            * testsuite/20_util/tuple/element_access/get_neg.cc: Adjust
            expected errors.
            * testsuite/20_util/tuple/element_access/101427.cc: New test.

[Bug libstdc++/101427] [11 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11/12 Regression] std::get |[11 Regression] std::get
                   |should refuse to compile if |should refuse to compile if
                   |type is provided and it is  |type is provided and it is
                   |duplicated in std::tuple    |duplicated in std::tuple

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk for now.

[Bug libstdc++/101427] [11 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:6dc150d9a036cbbed3c4dac6df1ce895b899d423

commit r11-8777-g6dc150d9a036cbbed3c4dac6df1ce895b899d423
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 14 20:14:14 2021 +0100

    libstdc++: Fix std::get<T> for std::tuple [PR101427]

    The std::get<T> functions relied on deduction failing if more than one
    base class existed for the type T.  However the implementation of Core
    DR 2303 (in r11-4693) made deduction succeed (and select the
    more-derived base class).

    This rewrites the implementation of std::get<T> to explicitly check for
    more than one occurrence of T in the tuple elements, making it
    ill-formed again. Additionally, the large wall of overload resolution
    errors described in PR c++/101460 is avoided by making std::get<T> use
    __get_helper<I> directly instead of calling std::get<I>, and by adding a
    deleted overload of __get_helper<N> for out-of-range N.

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/101427
            * include/std/tuple (tuple_element): Improve static_assert text.
            (__get_helper): Add deleted overload.
            (get<i>(tuple<T...>&&), get<i>(const tuple<T...>&&)): Use
            __get_helper directly.
            (__get_helper2): Remove.
            (__find_uniq_type_in_pack): New constexpr helper function.
            (get<T>): Use __find_uniq_type_in_pack and __get_helper instead
            of __get_helper2.
            * testsuite/20_util/tuple/element_access/get_neg.cc: Adjust
            expected errors.
            * testsuite/20_util/tuple/element_access/101427.cc: New test.

    (cherry picked from commit 17855eed7fc76b2cee7fbbc26f84d3c8b99be13c)

[Bug libstdc++/101427] [11 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

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

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

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 11.2, thanks for the report.