[llvm-bugs] [Bug 46678] New: LIBCXX_ABI_OPTIMIZED_FUNCTION: Incomplete return type in template argument for std::function fails to compile

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46678

            Bug ID: 46678
           Summary: LIBCXX_ABI_OPTIMIZED_FUNCTION: Incomplete return type
                    in template argument for std::function fails to
                    compile
           Product: libc++
           Version: 10.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dschuff at google.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

When using the optimized std::function ABI (LIBCXX_ABI_OPTIMIZED_FUNCTION)
and compiling the following:

#include <iostream>
#include <functional>
struct foo;
using fnc = std::function<foo(int)>;
struct bar {
    bar() {
        std::cout << "construct me" << std::endl;
    }
    fnc _f{nullptr};
};
int main(int argc, const char * argv[]) {
    // insert code here...
    std::cout << "Hello, World!\n";
    bar b{};
    return 0;
}

you get an error:
In file included from test.cc:1:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/iostream:37:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/ios:215:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/__locale:14:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/string:504:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/string_view:175:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/__string:56:
In file included from
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/algorithm:644:
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/functional:2051:16:
error: incomplete result type 'foo' in function definition
    static _Rp __call_empty(const __policy_storage*,
               ^
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/functional:2038:35:
note: in instantiation of member function
'std::__2::__function::__policy_invoker<foo (int)>::__call_empty' requested
here
    __policy_invoker() : __call_(&__call_empty) {}
                                  ^
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/functional:2090:5:
note: in instantiation of member function
'std::__2::__function::__policy_invoker<foo (int)>::__policy_invoker' requested
here
    __policy_func() : __policy_(__policy::__create_empty()) {}
    ^
/Users/dschuff/code/emr/emscripten-releases/emscripten/system/include/libcxx/functional:2276:5:
note: in instantiation of member function
'std::__2::__function::__policy_func<foo (int)>::__policy_func' requested here
    function(nullptr_t) _NOEXCEPT {}
    ^
test.cc:9:11: note: in instantiation of member function 'std::__2::function<foo
(int)>::function' requested here
    fnc _f{nullptr};
          ^
test.cc:3:8: note: forward declaration of 'foo'
struct foo;
       ^
1 error generated.


(this example uses emscripten, which currently uses libc++ 10.0 with some ABIv2
enabled)

If you instead use a complete type:
using fnc = std::function<void (int, foo&)>;
then it will compile.

On other platforms (and in libc++ using the stable std::function ABI) it also
works.

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