https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65106
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- See the definition of "modifiable lvalue" (6.3.2.1#1): "... if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const-qualified type". (Together with 6.5.16#2 "An assignment operator shall have a modifiable lvalue as its left operand.".) (It is however valid to pass arguments and return values of such types, notwithstanding references to assignment in such contexts; see DR#427.)
