Just the params
No. The semicolon separates those two sentences, so the implentation-defined manner is parallel to the entire sentence including the return value, which means the return value can also be implentation-defined.
Moreover, the 5.1.2.2.3 section explicitly tells you what will happen if the return value of main is not int.
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Actually, C++ enforces int return type on the main function, while C does not. C standards allow implentation-defined return type for the main function.