Did Pope Francis overstep any official Catholic boundary when he criticized Trump?

A better question would be, should Pope Francis consider Trump a Christian at all, since, in the Catechism, it is taught that "extra Ecclesiam nulla salus", or "outside the Church there is no salvation". Just as those who did not board the Ark were not saved. And Church history has been clear that Protestants are not considered Christian because they do not follow the teachings of Christ (in a Catholic point of view, of course).

For example, if the Pope was to call a protestant, who does not eat the flesh and drink the blood of Jesus (in the Eucharist), then how can he call them Christian if Christ said "Unless you eat my body and drink my blood, you will not have life in you." Someone who does not have life in them is not Christian, and not of Christ and are obviously lost.

Aside from doctrinal problems, should the Pope of the Catholic Church (this is the context) criticize anyone's Christianity?

Well, the Pope, as head of the Church, is believed to have been endowed by Christ with the keys to heaven and to bind and unbind ("bind and loose")... well, binding doctrines and laws. So he can exercise the authority to excommunicate people out of the Catholic Church, bu to someone like Trump, who is not inside the Church, it doesn't make much sense. He also has the authority to say someone is not being Christ-like or is a hypocrite, liar etc, if they are openly (which Trump has not been), but anyone has that power. In Catholic teaching, no one can be saved except through the Church, so to comment on Trump's Christianity is actually ignorant of the Pope.

On a side note, why does Pope Francis have a problem with building a wall, only when Trump says it, and not those who also agree one is needed (there already is a wall)? And more importantly, why didn't the Pope have a problem with Obama mocking the Sermon on the Mount, considered to be one of Christ's greatest speeches. I needn't mention the needles war crimes but I'm digressing.

891 of the Catechism states:

The Roman Pontiff, head of the College of Bishops, enjoys the infallibility of the virtue of his office, when, as supreme pastor and teacher of all the faithful--who confirms with all the brethren of the faith--he proclaims as a definitive act a doctrine pertaining to faith or morals.

It seems clear he is within his bounds.

Pope Francis did not run afoul of any Catholic policy by criticizing Trump but the Vatican was quick to issue an explanation that is a walk back from the perception left originally that the Pope considered Trump not Christian the quote below taken from the Daily mail:

THE GREAT WALL CLIMB-DOWN

'This wasn't in any way a personal attack, nor an indication of who to vote for.

The Pope has clearly said he didn't want to get involved in the electoral campaign in the US, and also said that he said what he said on the basis of what he was told [about Trump], hence giving him the benefit of the doubt.

The Pope said what we already know, if we followed his teaching and positions: We shouldn't build walls, but bridges.

He has always said that, continuously. He also said that in relation to migration in Europe many times.

So this is not a specific issue, limited to this particular case.

It's his generic view, coherent with the nature of solidarity from the Gospel.'

Vatican spokesman Father Federico Lombardi

Read the story at http://www.dailymail.co.uk/news/article-3454720/Popes-comments-Trump-not-personal-attack-papal-spokesman.html

There is no doctrine, policy or convention in the Catholic church that prevents a priest, or any church leader, from pronouncing on the rightness of some action or belief.

Historically, over the last thousand years or so, the Pope and other church leaders have pronounced on the policies and conduct of many world leaders. In recent times the Pope has condemned Apartheid, Communism, the Rwandan genocide, restrictions on the Cuban church, and much else.

It has to be pointed out that Pope Francis did not pronounce Trump a non-Christian, but pronounced the actions described to him by a questioner as those of a non-Christian, leaving open at least the possibility that the actions described in the question did not accurately reflect Trump's stance. Trump is also far from the only Presidential candidate both advocating wall-building and claiming to be Christian.