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>

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>>

R>(7.3) — Otherwise, no initialization is performed.

R>6.7.4 Indeterminate values
R>1 When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced ([expr.ass]). [ Note: Objects with static or thread storage duration are zero-initialized, see [basic.start.static]. — end note]

R>6.9.3.1 main function
R>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]).

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

W>A return statement in main has the effect of leaving the main function (destroying any objects with auto-
W>matic storage duration) and calling exit with the return value as the argument. If control reaches the end
W>of main without encountering a return statement, the effect is that of executing
W>return 0;