cmd/compile: regressed and non-informative error messages missing unimplemented methods when interface isn't satisfied

[odeke-em]

What version of Go are you using (go version)?

GoTip at 6f74ed0 aka Go1.18

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Not Applicable

What did you do?

Ran this program https://play.golang.org/p/Uan-Gj9JgbI or inlined below

package main

type Reader interface {
	Read() (int, error)
}

type rr int

var _ Reader = (*rr)(nil)

func main() {}

What did you expect to see?

./main.go:9:5: cannot use (*rr)(nil) (type *rr) as type Reader in assignment:
	*rr does not implement Reader (missing Read method)

What did you see instead?

./main.go:9:16: incompatible type: cannot use (*rr)(nil) (value of type *rr) as Reader value

Others and myself laboured in prior Go cycles to have informative error messages when interfaces weren't satisifed, and we wrote tests to lock this behavior in. We also got feedback from the community about the helpfulness of the prior user experience. I am surprised that this regressed quietly or that the tests perhaps got deleted.

Kindly cc-ing @griesemer @mdempsky

[ericlagergren]

Dupe of #48471?

[odeke-em]

Thank you @ericlagergren for pointing me to that issue! I implemented the prior diagnostics so looks like it is time for me to roll my sleeves up again soon :-)