A lambda’s return type can be deduced by the return value, so why can’t a function’s?

C++14 has this feature. You can test it with new versions of GCC or clang by setting the -std=c++1y flag.

Live example

In addition to that, in C++14 you can also use decltype(auto) (which mirrors decltype(auto) as that of variables) for your function to deduce its return value using decltype semantics.

An example would be that for forwarding functions, for which decltype(auto) is particularly useful:

template<typename function_type, typename... arg_types>
decltype(auto) do_nothing_but_forward(function_type func, arg_types&&... args) {
    return func(std::forward<arg_types>(args)...);

With the use of decltype(auto), you mimic the actual return type of func when called with the specified arguments. There’s no more duplication of code in the trailing return type which is very frustrating and error-prone in C++11.

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