Forward declaration and std::future

[Evgen Bodunov <evgen@globus.software>]

Hello everyone,

During development of valhalla routing project I stumbled upon compilation error on gcc + libstdc++ 11 and newer when std::future constructed from forward declared class. Live example is there: https://godbolt.org/z/rEx3Eonsz It builds on gcc prior to v11. and on any clang with libc++.

/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits: In instantiation of 'struct std::is_destructible<test_data>':
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21:   required from 'class std::promise<test_data>'
<source>:8:29:   required from here
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits:849:52: error: static assertion failed: template argument must be a complete class or an unbounded array
  849 |       static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits:849:52: note: 'std::__is_complete_or_unbounded<std::__type_identity<test_data> >((std::__type_identity<test_data>{}, std::__type_identity<test_data>()))' evaluates to false
In file included from <source>:1:
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future: In instantiation of 'class std::promise<test_data>':
<source>:8:29:   required from here
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21: error: static assertion failed: result type must be destructible
 1065 |       static_assert(is_destructible<_Res>{},
      |                     ^~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21: note: 'std::is_destructible<test_data>{}' evaluates to false
Compiler returned: 1

Is that static assert is really required? May be then it should correctly process forward declared types?

WBR,
Evgen Bodunov.