https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99518
--- Comment #1 from nickgray0 at brown dot edu --- I just looked up the standard and it seems GCC is correct to reject the code and Clang's implementation (which happily compiles the code) is incorrect. the compound requirement requires the parenthesized expression (which in this case, turns 'x' to int&) to satisfy the return type constraint. Sorry for the false alarm.
