cmd/compile: incorrect type set intersection computation [1.18 backport]

[gopherbot]

@griesemer requested issue #57486 to be considered for backport to the next 1.18 minor release.

This is now fixed (but not yet submitted). As @ianlancetaylor correctly noted, this should not have worked with Go 1.18 or Go 1.19.

@gopherbot please consider this for backport to 1.18 and 1.19. This is a compiler bug.

[gopherbot]

Change https://go.dev/cl/460015 mentions this issue: [release-branch.go1.18] go/types, types2: use strict comparability for type set intersection

[griesemer]

The primary reason for backporting would be for people who cannot (or don't want to) move to 1.20. People using 1.18 and and 1.19 can write code that the compiler accepts but that is in fact incorrect (though I don't have a simple example where such incorrectly compiled code can be used to exploit something - it'll take some ingenuity to find such an example).

The actual fix in the type checker is extremely straight-forward - most of the rest of the CL is simply for the test cases. I'm not worried too much about collateral damage.

My take: if there's no more point release for 1.18 and 1.19, then let's not do this. If there are point releases for other reasons, then let's include this fix.

[heschi]

We aren't going to do another minor release for 1.18. Closing this one.