Would not be surprising

Given his boastful character and inept regard for details, it would not be very surprising at all if Trump did disclose details he shouldn't have to Russians (or anyone he is trying to impress).   The only surprise would be that he had paid enough attention to recall the detail he shouldn't be sharing.  From Axios:

President Trump revealed highly classified information in an Oval Office meeting with Russian Foreign Secretary Sergey Lavrov and Ambassador Sergei Kislyak, potentially damaging relations with a key source of intelligence on ISIS, according to the Washington Post.

• A source told the Post Trump discussed material with the highest level of classification, and "revealed more information to the Russian ambassador than we have shared with our own allies."
• Trump seemed to be "boasting about his inside knowledge of the looming threat" before describing a specific ISIS plot and where it was detected, per the report. The intelligence-sharing system through which the U.S. learned of the plot is incredibly sensitive.

Update:  despite McMaster trying to throw cold water on the story, this detail at the end of the Wapo report seems to indicate there's something to it:

Senior White House officials appeared to recognize quickly that Trump had overstepped and moved to contain the potential fallout. Thomas P. Bossert, assistant to the president for homeland security and counterterrorism, placed calls to the directors of the CIA and the NSA, the services most directly involved in the intelligence-sharing arrangement with the partner.

Update 2:   just another shouty evening in the White House:

7:24PM: Chief strategist Steve Bannon and top communications officials Mike Dubke, Sarah Sanders, and Sean Spicer walk into cabinet room, per reporters on Twitter who then hear yelling from the meeting.

 Update 3:  more on the shouting:

Twitter (and much of the media) is going berserk over this:   Trump is going to be very, very upset about it all....