A simpler version of lib.go that doesn't require an import but still recreates the problem:

// lib/lib.go
package lib

type Map[K comparable, V any] map[K]V

type FMap[K, V comparable] Map[K, V]

func (m FMap[K, V]) Flip() FMap[V, K] {
	out := make(FMap[V, K])
	for k, v := range m {
		out[v] = k
	}

	return out
}

type MyType uint8

const (
	FIRST MyType = iota
)

var typeStrs = FMap[MyType, string]{
	FIRST:  "FIRST",
}

func (self MyType) String() string {
	str, exists := typeStrs[self]
	if !exists {
		return "UNKNOWN"
	}

	return str
}

CC @randall77 @mdempsky

@gopherbot Please open a backport to 1.18

This looks like a compiler crash on valid code.

Backport issue(s) opened: #52286 (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.

Repro on playground: https://go.dev/play/p/lFHvAeGwc1g?v=gotip

I'm looking at this, as part of my "learn generics by playing with bugs", and I see (with two debugging booleans set and -W=2)

# bug/lib
...
=== Finalized dictionary .dict.FMap[string,"".MyType].Flip

Where the missing dictionary is bug/lib..dict.FMap[string,bug/lib.MyType].Flip.

Do I read this correctly as an export/import problem?

Still slogging, intermittent with other work, but with gdlv on the failed compile, it is compiling
(*FMap[string,bug/lib.MyType]).Flip -- so this is an an export problem (?), for the initial export (and not, say, a re-export of something in bug with inlining). Perhaps, actually a naming problem, in some way similar to https://go-review.googlesource.com/c/go/+/391574 ?

Error messages suggest however that I am compiling package "bug" and that perhaps the value of the current function is stale?

The bug does not reproduce when inlining is turned off.
The bug does not reproduce with GOEXPERIMENT=unified.

Further work. A simpler version of lib/lib.go:

// lib/lib.go
package lib

type FMap[K comparable, V comparable] map[K]V

//go:noinline
func (m FMap[K, V]) Flip() FMap[V, K] {
	out := make(FMap[V, K])
	return out
}

type MyType uint8

const (
	FIRST MyType = 0
)

var typeStrs = FMap[MyType, string]{
	FIRST: "FIRST",
}

func (self MyType) String() string {
	return typeStrs[self]
}

// func F() FMap[string, MyType] {
// 	return typeStrs.Flip()
// }

Uncomment F at the bottom to make it pass; I think this is an export problem, where the exported inline for MyType.String() includes a reference to typeStrs (type FMap[MyType, string]) which in turn has a method Flip() which mentions the type FMap[string, MyType] which also has a method Flip. I haven't figured out where/how this is all supposed to be exported, only that it is not.

Change https://go.dev/cl/405118 mentions this issue: cmd/compile: be sure to export types mentioned in f.i.g. method signature

Change https://go.dev/cl/405543 mentions this issue: cmd/compile: be sure to export types mentioned in f.i.g. method signature