President Trump’s theory of government spending isn’t complicated: Spending that he doesn’t like is bad and spending that he likes is good. This approach isn’t unique to Trump, of course, but Trump does seem to be more explicit about it.

There was his tweet Wednesday morning, for example, in which he lamented that “so much money has been poured down the drain, for so many years,” but that when it came to border security — that is, his proposed wall on the border with Mexico — “the Democrats fight to the death.”

The Republicans have had a few throw-downs in recent years, too, one might recall, including shutting down the government in 2013 in a futile effort to block implementation of Obamacare. But, you know. That was then.

What’s fascinating about Trump’s tweet is that juxtaposition: Federal revenue poured down the drain while Democrats nickel-and-dime him on something he wants. You waste all this money and then you’re going to whine about an estimated $18 billion for a border wall? What gives?

The irony here is that Trump has done a bit of drain-pouring himself — not to mention some serious shutting off the tap.

Consider, for example, tax receipts in the fiscal year that ended Oct. 1.

Calculations from the Congressional Budget Office show that despite the tax cuts signed into law in December 2017 — a few months into fiscal 2018 — the government saw about $96 billion more from individual income taxes in fiscal 2018 than in fiscal 2017.

Payroll taxes increased too, but more subtly. Compared with fiscal 2017, payroll tax receipts increased by $9 billion.

One of the critiques of the tax bill, of course, was that the benefits disproportionately went to corporations. And, lo, in fiscal 2018, receipts from corporate taxes dropped by $92 billion.

Enough to pay for the wall more than five times over.

Corporate tax receipts have been on a downward trend for a while. From fiscal 2015 to 2016, receipts dropped $44 billion. From 2016 to 2017, $3 billion more. The drop last year, though, was substantially different.

Receipts dropped “from 1.5 percent to 1.0 percent of GDP,” the CBO noted. “That percentage of GDP is the lowest recorded since 2009” — when the recession was dampening the economy — “and it is half of the 50-year average of 2.0 percent of GDP.”

Trump’s pitch for slashing corporate taxes was that the companies would then reinvest that money in jobs, salaries and expansion. Here’s what job growth looked like over that same period.

The two lines began to separate in about March. The result? Job growth in fiscal 2018 surpassed 2017 — ending up about where growth had been in 2015 and 2016.

An analysis by The Post last week looked at where most of the corporate tax cuts went. Investments increased 19 percent to $475 billion. Research and development was up 34 percent to $175 billion.

Stock buybacks — buying stock from shareholders instead of investing in the company — jumped to $579 billion. Dividends rose to $420 billion. Both are records.

$579 billion, incidentally, could buy 32 border walls.