cmd/compile: unsafe.Offsetof returns incorrect value in embedded struct with type parameters [1.18 backport]

[gopherbot]

@randall77 requested issue #53137 to be considered for backport to the next 1.18 minor release.

@gopherbot please open a backport issue for 1.18.

[gopherbot]

Change https://go.dev/cl/409494 mentions this issue: [release-branch.go1.18] cmd/compile: fix wrong unsafe.Offsetof evaluation inside generic function

[dmitshur]

@randall77 Thanks for requesting the backport. Can you please include a brief rationale per https://go.dev/wiki/MinorReleases (third paragraph)—thanks.

[randall77]

This causes the compiler to incorrectly compile some unsafe.OffsetOf calls in generic types. It's possible to work around in some cases, but probably not all (e.g. using unsafe.OffsetOf on types in a package you don't control).