[llvm-bugs] [Bug 51359] New: trunk libcxx does not build with g++ 9.3.0

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51359

            Bug ID: 51359
           Summary: trunk libcxx does not build with g++ 9.3.0
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: brian.sumner at amd.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Trying a build of a recent monorepo commit with DLLVM_ENABLE_PROJECTS including
libcxx with the standard compiler on my Ubuntu 20.04 system:

$ /usr/bin/c++ --version
c++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

results in a build failure.  This appears to be a g++ bug, because the build
sets -std=c++2a, but g++ is complaining that a destructor cannot be constexpr. 
However, if LLVM is supposed to build with gcc > 5.1.0, then libcxx may need to
be fixed.

Here is the build output with the failure:

make[2]: Entering directory '/git/build-llvm'
[ 42%] Building CXX object
projects/libcxx/src/CMakeFiles/cxx_static.dir/optional.cpp.o
cd /git/build-llvm/projects/libcxx/src && /usr/bin/c++ -DNDEBUG -D_DEBUG
-D_GNU_SOURCE -D_LIBCPP_BUILDING_LIBRARY
-D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
-I/git/build-llvm/projects/libcxx/src -I/git/llvm-monorepo/libcxx/src
-I/git/build-llvm/include -I/git/llvm-monorepo/llvm/include
-I/git/build-llvm/projects/libcxx/include/c++build -fPIC
-fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall
-Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual
-Wno-missing-field-initializers  -Wno-long-long -Wimplicit-fallthrough
-Wno-maybe-uninitialized -Wno-class-memaccess -Wno-redundant-move
-Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor
-Wsuggest-override -Wno-comment -Wmisleading-indentation -g -fPIC
-DLIBCXX_BUILDING_LIBCXXABI -nostdinc++ -fvisibility-inlines-hidden -Wall
-Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long
-Werror=return-type -Wextra-semi -Wundef -Wno-literal-suffix -Wno-c++14-compat
-Wno-noexcept-type -Wno-suggest-override -Wno-error
-I/git/build-llvm/include/c++/v1 -std=c++2a -o
CMakeFiles/cxx_static.dir/optional.cpp.o -c
/git/llvm-monorepo/libcxx/src/optional.cpp
In file included from /git/build-llvm/include/c++/v1/__memory/shared_ptr.h:25,
                 from
/git/build-llvm/include/c++/v1/__functional/function.h:20,
                 from /git/build-llvm/include/c++/v1/functional:500,
                 from /git/build-llvm/include/c++/v1/optional:154,
                 from /git/llvm-monorepo/libcxx/src/optional.cpp:9:
/git/build-llvm/include/c++/v1/__memory/unique_ptr.h:746:50: warning: type
attributes ignored after type is already defined [-Wattributes]
  746 | template <class _Tp> struct _LIBCPP_TEMPLATE_VIS hash;
      |                                                  ^~~~
In file included from
/git/build-llvm/include/c++/v1/__functional/function.h:20,
                 from /git/build-llvm/include/c++/v1/functional:500,
                 from /git/build-llvm/include/c++/v1/optional:154,
                 from /git/llvm-monorepo/libcxx/src/optional.cpp:9:
/git/build-llvm/include/c++/v1/__memory/shared_ptr.h:1710:50: warning: type
attributes ignored after type is already defined [-Wattributes]
1710 | template <class _Tp> struct _LIBCPP_TEMPLATE_VIS hash;
      |                                                  ^~~~
In file included from /git/build-llvm/include/c++/v1/__availability:13,
                 from /git/build-llvm/include/c++/v1/optional:149,
                 from /git/llvm-monorepo/libcxx/src/optional.cpp:9:
/git/build-llvm/include/c++/v1/__config:1054:41: error: a destructor cannot be
'constexpr'
1054 | #  define _LIBCPP_CONSTEXPR_AFTER_CXX17 constexpr
      |                                         ^~~~~~~~~
/git/build-llvm/include/c++/v1/optional:224:5: note: in expansion of macro
'_LIBCPP_CONSTEXPR_AFTER_CXX17'
  224 |     _LIBCPP_CONSTEXPR_AFTER_CXX17 ~__optional_destruct_base()
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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