General

Misc

function arguments are evaluated in undefined order
local objects are destroyed in order reversed to construction
inline before function return type is only a suggestion, not enforcement

Trailing Return Type (since C++11)

Works both for functions and function templates

auto f() -> int

template<typename T1, typename T2>
auto g(T1 a, T2 b) -> decltype(a + b)

the decl-specifier-seq must contain the keyword auto
useful when return type depends on arguments

Return Type Deduction (since C++14)

If the decl-specifier-seq of the function declaration contains the keyword auto, trailing return type may be omitted, and the return type will be deduced from the type of the expression used in the return statement.

auto f() { return 5; }

if the return type does not use decltype(auto), the deduction follows the rules of template argument deduction
if the return type is decltype(auto), the return type is as what would be obtained if the expression used in the return statement were wrapped in decltype
if there are multiple return statements, they must all deduce to the same type
virtual functions and coroutines cannot use return type deduction
if the return statement uses a brace-init-list, deduction is not allowed

Abbreviated function template (since C++20)

If placeholder type (auto/Concept auto) appears in function or function template parameter list, then declaration declares a function template, and one invented template parameter for each placeholder is appended to the template parameter list.

int f(auto a, auto b)

is the same as:

template<typename T1, typename T2>
int f(T1 a, T2 b)