Functions that have trailing returns are disallowed, except for those using decltype specifiers and lambda with otherwise unutterable return types.

For example:

// No warning
int add_one(const int arg) { return arg; }

// Warning
auto get_add_one() -> int (*)(const int) {
  return add_one;

Exceptions are made for lambdas and decltype specifiers:

// No warning
auto lambda = [](double x, double y) -> double {return x + y;};

// No warning
template <typename T1, typename T2>
auto fn(const T1 &lhs, const T2 &rhs) -> decltype(lhs + rhs) {
  return lhs + rhs;

See the features disallowed in Fuchsia at https://fuchsia.dev/fuchsia-src/development/languages/c-cpp/cxx?hl=en