Is a placeholder return type allowed for main?

Is the following valid in C++14?

auto main()
{
    // ...
}

Will the return type deduce to int even though we don't need to use an explicit return 0;?

No, it isn't allowed. Paragraph 7.1.6.4/10 of the C++14 Standard Draft N3690 specifies:

If a function with a declared return type that uses a placeholder type has no return statements, the return type is deduced as though from a return statement with no operand at the closing brace of the function body. [...]

This means that omitting a return statement in main() would make its type void.

The special rule introduced by paragraph 3.6.1/5 about flowing off the end of main() specifies:

[...] If control reaches the end of main without encountering a return statement, the effect is that of executing

return 0;

The wording says that the "effect" during the execution of the program is the same as though a return 0 was present, not that a return statement will be added to the program (which would affect type deduction according to the quoted paragraph).

EDIT:

There is a Defect Report for this (courtesy of Johannes Schaub):

Proposed resolution (November, 2013):

Change 3.6.1 [basic.start.main] paragraph 2 as follows:

An implementation shall not predefine the main function. This function shall not be overloaded. It shall have a declared return type of type int, but otherwise its type is implementation-defined. All implementations An implementation shall allow both

• a function of () returning int and
• a function of (int, pointer to pointer to char) returning int

as the type...