https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82899
--- Comment #2 from Antony Polukhin <antoshkka at gmail dot com> --- Looks like [class.ctor] paragraph 14 covers this case: "During the construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor’s this pointer, the value of the object or subobject thus obtained is unspecified."