prog.cpp: In function ‘int main()’:
prog.cpp:20:7: error: expected primary-expression before ‘auto’
if ((auto a = foo()).isValid()) // а так не работает
^~~~
prog.cpp:20:7: error: expected ‘)’ before ‘auto’
prog.cpp:24:2: error: expected ‘)’ before ‘return’
return 0;
^~~~~~