HANOI, Vietnam — Officially, the top world leaders who gather Thursday at the Group of 7 summit meeting will talk about shared concerns, like global trade or the Islamic State. But their private discussions are likely to cover a topic that is not on the agenda: Donald J. Trump.

President Obama now hears questions in his meetings with world leaders about whether Mr. Trump has a realistic shot at becoming president.

For months, Mr. Obama has answered those questions with an emphatic “no.”

“I continue to believe Mr. Trump will not be president,” Mr. Obama said in February at the end of the Association of Southeast Asian Nations summit meeting in California.

But this week’s summit meeting in Japan is the first among major allies since Mr. Trump moved decisively toward securing the Republican nomination, and Mr. Obama’s counterparts are likely to want more detail in his explanations of an election that has prompted fascination and apprehension overseas.