R>>#include <iostream>

R>>int x;

R>>int main() {
R>>    int y;
R>>    std::cout << x << std::endl;    // OK
R>>    std::cout << y << std::endl;    // warning: 'y' is used uninitialized [-Wuninitialized]
R>>} // а где warning no return value ?
R>>

6.9.3.1 main function
5 A return statement ([stmt.return]) in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std​::​exit with the return value as the argument. If control flows off the end of the compound-statement of main, the effect is equivalent to a return with operand 0 (see also [except.handle]).

2 . . .
Otherwise, flowing off the end of a function other than main or a coroutine ([dcl.fct.def.coroutine]) results in undefined behavior.

R>>#include <iostream>

R>>int x;

R>>int main() {
R>>    int y;
R>>    std::cout << x << std::endl;    // OK
R>>    std::cout << y << std::endl;    // warning: 'y' is used uninitialized [-Wuninitialized]
R>>} // а где warning no return value ?
R>>

6.9.3.1 main function
5 A return statement ([stmt.return]) in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std​::​exit with the return value as the argument. If control flows off the end of the compound-statement of main, the effect is equivalent to a return with operand 0 (see also [except.handle]).

8.7.3 The return statement
2 . . . Otherwise, flowing off the end of a function other than main or a coroutine ([dcl.fct.def.coroutine]) results in undefined behavior.