design/36460/registry_incompatibility.txt

# This example illustrates a case in which a seemingly-irrelevant requirements
# in one module fixes an incompatibility with a package in another.
#
# Two packages that do not import each other may interact via a third (mutual)
# dependency, so their module requirements must be respected even if those
# requirements do not directly correspond to package imports.


# Before discovering the import of package c, we depend on b v1.0.0.

$ go1.14rc1 list -m b
b v1.0.0 => ./b0


# When we import package c, we must update the selected version of b according
# to c's requirements, _even though_ neither package imports the other.
# Otherwise, the program will panic at run time.

$ go1.14rc1 build c
go: found c in c v1.0.0

$ go1.14rc1 list -m b
b v1.0.1 => ./b1


-- go.mod --
module main

go 1.15

require (
	a v1.0.0
	b v1.0.0
	d v1.0.0
)

replace (
	a v1.0.0 => ./a
	b v1.0.0 => ./b0
	b v1.0.1 => ./b1
	c v1.0.0 => ./c
	d v1.0.0 => ./d
)
-- main.go --
package main

import (
	_ "a"

	// Adding an import of c should increase the selected version of b to 1.0.1
	// in order to avoid a registration collision caused by a bug in b 1.0.0.
	// Module b is not otherwise relevant to package c or its containing module.
	_ "c"
)

func main() {}
-- a/go.mod --
module a

go 1.15

require b v1.0.0
-- a/a.go --
package a

import _ "b"
-- b0/go.mod --
module b

go 1.15

require d v1.0.0
-- b0/b.go --
package b

import "d"

func init() { d.Register("c") }  // This is a bug that breaks package c.
-- b1/go.mod --
module b

go 1.15

require d v1.0.0
-- b1/b.go --
package b

import "d"

func init() { d.Register("b") }  // The bug has been fixed.
-- c/go.mod --
module c

require (
	b v1.0.1
	d v1.0.0
)
-- c/c.go --
package c

import "d"

func init() { d.Register("c") }

-- c/tidy/tidy.go --
// +build tidy-only

package tidy

// Maintain a false dependency on b v1.0.1, to prevent b v1.0.0
// from erroneously registering itself with our path if anything
// that imports us also imports package b.
import _ "b"
-- d/go.mod --
module d

go 1.15
-- d/d.go --
package d

var registered = map[string]bool{}

func Register(key string) {
	if registered[key] {
		panic("duplicate registation for  " + key)
	}
	registered[key] = true
}