Error(s):

source_file.cpp:6:52: error: invalid use of incomplete type ‘struct S’
     static auto bar() -> decltype(std::declval<S>().foo());
                                                    ^
source_file.cpp:3:8: error: forward declaration of ‘struct S’
 struct S
        ^
source_file.cpp:6:52: error: invalid use of incomplete type ‘struct S’
     static auto bar() -> decltype(std::declval<S>().foo());
                                                    ^
source_file.cpp:3:8: error: forward declaration of ‘struct S’
 struct S
        ^
source_file.cpp:7:27: error: ‘bar’ was not declared in this scope
     void baz(decltype(bar()));
                           ^
source_file.cpp:7:27: error: ‘bar’ was not declared in this scope