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

[kazu]

I checked in 1.18.2 / go1.19-52f68efa45.

https://go.dev/play/p/nSbR4HtjOSv

[kazu]

https://go.dev/play/p/9Joi2y5p8oE

[ianlancetaylor]

CC @randall77 @mdempsky

[cuonglm]

It seems that when instantiating generic function, all implicit dot operations are resolved, so unsafe.Offsetof(d.B) becomes unsafe.Offsetof(d.Embedded.B) at the time we call evalunsafe. Thus the base selector is d.Embedded instead of d.

Maybe we just need to clobber any implicit dot, something like this should work:

// Remember base of selector to find it back after dot insertion.
// Since r->left may be mutated by typechecking, check it explicitly
// first to track it correctly.
sel.X = Expr(sel.X)
sbase := sel.X

var clobberBase func(n ir.Node) ir.Node
clobberBase = func(n ir.Node) ir.Node {
	if x, ok := n.(*ir.SelectorExpr); ok && x.Implicit() {
		return clobberBase(x.X)
	}
	return n
}
sbase = clobberBase(sbase)

[gopherbot]

Change https://go.dev/cl/409355 mentions this issue: cmd/compile: fix wrong unsafe.Offsetof evaluation inside generic function

[cuonglm]

Should we backport this cc @ianlancetaylor @randall77 ?

[randall77]

@gopherbot please open a backport issue for 1.18.

[gopherbot]

Backport issue(s) opened: #53159 (for 1.18).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.