'#pragma GCC system_header' changes vs. 'ignore' for 'std::tr1::tie'

[Thomas Schwinge]

Hi!

When building libstdc++-v3 with '--enable-werror', the build currently fails:

    [...]
    In file included from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/functional:42,
                     from ../../../../../source-gcc/libstdc++-v3/src/c++98/hash_tr1.cc:27:
    [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/tuple:426:21: error: ‘std::tr1::{anonymous}::ignore’ defined but not used [-Werror=unused-variable]
      426 |     _Swallow_assign ignore;
          |                     ^~~~~~
    cc1plus: all warnings being treated as errors
    make[5]: *** [Makefile:658: hash_tr1.lo] Error 1
    [...]
    In file included from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/functional:42,
                     from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/ext/rope:61,
                     from ../../../../../source-gcc/libstdc++-v3/src/c++98/ext-inst.cc:29:
    [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/tuple:426:21: error: ‘std::tr1::{anonymous}::ignore’ defined but not used [-Werror=unused-variable]
      426 |     _Swallow_assign ignore;
          |                     ^~~~~~
    cc1plus: all warnings being treated as errors
    make[5]: *** [Makefile:658: ext-inst.lo] Error 1
    make[5]: Target 'all' not remade because of errors.
    make[5]: Leaving directory '[...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/src/c++98'
    [...]
    In file included from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/functional:42,
                     from ../../../../../source-gcc/libstdc++-v3/src/c++11/cxx11-hash_tr1.cc:32:
    [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/tuple:426:21: error: ‘std::tr1::{anonymous}::ignore’ defined but not used [-Werror=unused-variable]
      426 |     _Swallow_assign ignore;
          |                     ^~~~~~
    cc1plus: all warnings being treated as errors
    make[5]: *** [Makefile:681: cxx11-hash_tr1.lo] Error 1
    make[5]: Target 'all' not remade because of errors.
    make[5]: Leaving directory '[...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/src/c++11'
    [...]

Should we just mute this specific '-Wunused-variable', or (if that makes
any sense) apply to 'tr1/tuple' changes similar to the 'std/tuple' onces
of commit 1489e281b549af8a76a415556c07e147df933a44
'libstdc++: Implement P2968R2 "Making std::ignore a first-class object"'?
Doing just:

    --- libstdc++-v3/include/tr1/tuple
    +++ libstdc++-v3/include/tr1/tuple
    @@ -409,22 +409,6 @@ namespace tr1
         {
           return tuple<_Elements&...>(__args...);
         }
    -
    -  // A class (and instance) which can be used in 'tie' when an element
    -  // of a tuple is not required
    -  struct _Swallow_assign
    -  {
    -    template<class _Tp>
    -      _Swallow_assign&
    -      operator=(const _Tp&)
    -      { return *this; }
    -  };
    -
    -  // TODO: Put this in some kind of shared file.
    -  namespace
    -  {
    -    _Swallow_assign ignore;
    -  } // anonymous namespace
     }
     
     _GLIBCXX_END_NAMESPACE_VERSION

... is not sufficient:

    [-PASS:-]{+FAIL:+} tr1/6_containers/headers/tuple/types_std_tr1.cc  -std=gnu++17 (test for excess errors)

    [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/headers/tuple/types_std_tr1.cc:24: error: 'ignore' has not been declared in 'std::tr1'

    [-PASS:-]{+FAIL:+} tr1/6_containers/tuple/cons/big_tuples.cc  -std=gnu++17 (test for excess errors)
    [-PASS:-]{+UNRESOLVED:+} tr1/6_containers/tuple/cons/big_tuples.cc  -std=gnu++17 [-execution test-]{+compilation failed to produce executable+}

    [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/tuple/cons/big_tuples.cc: In function 'int main()':
    [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/tuple/cons/big_tuples.cc:85: error: 'ignore' was not declared in this scope; did you mean 'std::ignore'?
    In file included from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/stl_pair.h:62,
                     from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/utility:71,
                     from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/tuple:44,
                     from [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/tuple/cons/big_tuples.cc:22:
    [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/utility.h:309: note: 'std::ignore' declared here

    [-PASS:-]{+FAIL:+} tr1/6_containers/tuple/creation_functions/tie2.cc  -std=gnu++17 (test for excess errors)
    [-PASS:-]{+UNRESOLVED:+} tr1/6_containers/tuple/creation_functions/tie2.cc  -std=gnu++17 [-execution test-]{+compilation failed to produce executable+}

    [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/tuple/creation_functions/tie2.cc: In function 'int main()':
    [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/tuple/creation_functions/tie2.cc:32: error: 'ignore' was not declared in this scope; did you mean 'std::ignore'?
    In file included from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/stl_pair.h:62,
                     from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/utility:71,
                     from [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/tr1/tuple:44,
                     from [...]/source-gcc/libstdc++-v3/testsuite/tr1/6_containers/tuple/creation_functions/tie2.cc:20:
    [...]/build-gcc/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/utility.h:309: note: 'std::ignore' declared here


Grüße
 Thomas