$ go run -compiler=gccgo x.go
FAIL:
*interface { a.i }
*interface { b.i }

$ head -20 go.mod x.go a/a.go b/b.go
==> go.mod <==
module m

go 1.20

==> x.go <==
package main

import (
	"fmt"
	"m/a"
	"m/b"
)

func main() {
	if a.X != b.X {
		fmt.Printf("FAIL:\n%T\n%T\n", a.X, b.X)
	}
}

==> a/a.go <==
package a

type i interface{ M() interface{ i } }

var X interface{} = (*interface{ i })(nil)

==> b/b.go <==
package b

type i interface{ M() interface{ j } }
type j interface{ M() interface{ i } }

var X interface{} = (*interface{ i })(nil)