> For example, HVDC. Interconnect and buy power from somebody with more sun.

Who is Japan interconnecting with, or any other country that doesn't trust its neighbors? What is Canada supposed to do when it's ~6000 km from the equator and might not want to rely on the US for electricity regardless?

> Or just overbuild solar by a lot. It's cheap, so chances are having too much of it still works out economically.

Solar is cheap per kWh but those kWh come disproportionately in the sunnier months of the year at any non-equatorial latitude. To build enough for January you'd then have oversupply and a price of zero for the nine months out of the year when you have the most output, requiring you to make back the entire cost in the three months when solar output is lowest. Then you're only getting paid anything for e.g. 12.5% of the kWh you generate (the 25% of the months that have 50% of the average output) which means you need the price during those months to be 8x the average price you need to break even, but then you're not cheaper than existing alternatives. And that's before you even deal with nights or cloudy winter days.

It obviously makes sense to use solar to reduce the need for natural gas plants during hot summer days with a lot of air conditioning demand, or for charging electric cars that can hold off a couple days if it's cloudy. It equally obviously doesn't make sense to try to generate 100% of electricity from the same intermittent source whose output is regionally correlated by season and weather systems.