Donald Trump flew to Mexico yesterday for a meeting with President Enrique Peña Nieto.

After the meeting, Trump said that he and the president did not discuss the question of who would pay for Trump’s proposed wall on the US–Mexican border. Then Trump went to a rally in Phoenix where he delivered a deranged, ranty speech about the evils of immigration and reiterated that Mexico will pay for the wall. Early this morning on Twitter, he again made his point clear.

Mexico will pay for the wall! — Donald J. Trump (@realDonaldTrump) September 1, 2016

Peña Nieto, meanwhile, was under fire in Mexico for not having stood up to Trump adequately in their post-meeting press conference. But his staff put the word out that during the meeting he said Mexico would not pay for the construction of the wall, a stance he reiterated on Twitter this afternoon in reply to Trump.

Repito lo que le dije personalmente, Sr. Trump: México jamás pagaría por un muro. https://t.co/IJNVe0XepY — Enrique Peña Nieto (@EPN) September 1, 2016

This means, basically, "Repeating what I told you in person, Mr. Trump: Mexico will never pay for a wall."

To be fussy and technical about it, my view is that given the current low interest rate environment, the question of who actually finances the wall’s construction is fairly secondary. The problem with the wall, economically speaking, is that building both the wall and the necessary supporting infrastructure (much of the border passes through wilderness or desert areas that currently lack the roads you would need to ship in the components) would require an enormous amount of manpower, concrete, steel, and other construction materials.

There are some idle resources currently in the United States, but the reality is that only so many trained construction workers and so much construction equipment exists in the US. Consequently, work on the wall and the supporting infrastructure would necessarily come at the expensive of other large-scale civil engineering undertakings, whether Mexico pays for it or not.

The Republican Party’s presidential nominee and the incumbent president of Mexico are having a fight on Twitter, because this is apparently how the world works now.

Watch: Trump called Peña Nieto a friend on Wednesday