cmd/compile: restore nested.go test case

[mdempsky]

In CL 421821, I've commented out part of the test in nested.go, because it's incredibly contrived and runs into an awkward reentrancy issue with unified IR and shape types. I expect the issue will go away once #46731 is implemented, and unified IR updated to take advantage of it. This is a reminder issue to make sure that happens before Go 1.20.

[mdempsky]

Confirmed that changing this line:

go/src/cmd/compile/internal/noder/writer.go

Line 793 in 6dc7b06

w.typExpr(decl.Type)

to w.typ(named.Underlying()) fixes nested.go. (And does not affect the export data format.)

We can make that change as soon as the compiler no longer needs to support:

//go:notinheap
type A /* some type */

type B A // expected to also be NIH

(We don't need to have necessarily removed //go:notinheap entirely yet, just decided that we're not going to support that problematic use case anymore; in particular, removed any test cases that expect that.)

[gopherbot]

Change https://go.dev/cl/424854 mentions this issue: cmd/compile: restore test/nested.go test cases

[mdempsky]

We reverted the fix.

[gopherbot]

Change https://go.dev/cl/426335 mentions this issue: cmd/compile: fix unified IR shapifying recursive instantiated types

[gopherbot]

Change https://go.dev/cl/455279 mentions this issue: cmd/compile: restore test/nested.go test cases