cmd/compile: ICE on embedded method expression with receiver parameter of derived type

[mdempsky]

This program is valid, but ICEs with the nounified frontend: https://go.dev/play/p/54ohNsbev55

It appears to be a bad interaction between method expressions, receiver parameters of derived type, and method embedding.

It currently works with GOEXPERIMENT=unified. Marking for Go 1.20, because I expect enabling unified IR by default is how we'll fix this issue.

/cc @golang/compiler

[cuonglm]

Unified IR is enabled now, this is fixed at tip.

Should we close this one?

@mdempsky @randall77

[mdempsky]

Yeah, this never worked, so no need for backports. There's a regress test for this in typeparam/mdempsky/20.go too.