Why can't auto be return type of a function? [duplicate]

This question already has an answer here:

• Omit return type in C++11 7 answers


• A lambda's return type can be deduced by the return value, so why can't a function's? 5 answers

My question is, why can't the return type of a function be deduced ? , or more simply why the following code gives error :

auto myfunc(int a)
{
int a = 12;
return a;
}

Why this isn't valid ?