https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83373
--- Comment #8 from Martin Sebor <msebor at gcc dot gnu.org> --- GCC treats the last member array of a [sub]object as a flexible array member only if it doesn't know the size of the whole object (the subobject can be a member of a larger struct or an element of an array). When it knows that the [sub]object is, in fact, the whole object, it treats the array as an ordinary one. In that case the optimization kicks in and the warning is avoided. With that, since in your example in comment #6 msg is a whole object GCC doesn't warn with the patch. If msg were a reference passed to func() then GCC would treat the array as a flexible array member and the optimization would not take place. But in that case, memcpy treats a pointer to such an array as pointing to an object of potentially larger size and so no warning is issued either.
