clang-38 produces an errors on 10.3, but succeeds on 11.0

[Yuri]

Getting the error while trying to update graphics/aseprite 
(https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213756).

Same clang-38 compiler, same port directory, but the outcome is different.

On 10.3 this error is produced:
 > 
/usr/ports/graphics/aseprite/work/aseprite-1.1.9/src/observable/obs/slot.h:51:17: 
error: no matching constructor for initialization of 'std::function<void 
()>'

 >   slot(F&& f) : f(std::forward<F>(f)) { }


On 11.0 build succeeds with both base and port clang-38.


What can be causing this error on 10.3?


Yuri