target: target,
		list:   downgraded,
		Reqs:   reqs,
	})
	if err != nil {
		return nil, err
	}
	actualVersion := make(map[string]string, len(actual))
	for _, m := range actual {
		actualVersion[m.Path] = m.Version
	}

	downgraded = downgraded[:0]
	for _, m := range list {
		if v, ok := actualVersion[m.Path]; ok {
			downgraded = append(downgraded, module.Version{Path: m.Path, Version: v})
		}

B3: D2
C1: B2.hidden
C2: D2
D1:
D2:
build A1: A1 B3 D2
downgrade A1 D1: A1 B1 D1 E1
build A: A B3 C2 D2
downgrade A D1: A B2.hidden C1 D1

# Both B3 and C3 require D2.
# If we downgrade D to D1, then in isolation B3 would downgrade to B1,
# and C3 would downgrade to C1.
# But C1 requires B2.hidden, and B1 requires C2.hidden, so we can't
# downgrade to either of those without pulling the other back up a little.
#

		// warning: v2->v3 ha upgrades not currently supported.
		{"ha-v3-v3-up", 3, "3.0.17/etcd3", "3.1.12/etcd3", "https", ""},

		// downgrades
		{"v3-v3-down", 1, "3.1.12/etcd3", "3.0.17/etcd3", "https", ""},

		// warning: ha downgrades not yet supported.
	}

	for _, m := range migrations {
		t.Run(m.title, func(t *testing.T) {
			start := mustParseEtcdVersionPair(m.startVersion)

				// If this conflict was just one step in a longer chain of downgrades,
				// then we would want to keep going past it until we find a version
				// that doesn't have that problem. However, we only want to downgrade
				// away from an *existing* requirement if we can confirm that it actually
				// conflicts with mustSelect. (For example, we don't want
				// 'go get -u ./...' to incidentally downgrade some dependency whose

        "ReadWriteC -> ReadWriteA",
        "ReadWriteB -> ReadWriteC",
        "ReadWriteA -> ReadWriteB");
  }

  public void testWriteToReadLockDowngrading() {
    writeLockA.lock(); // writeLockA downgrades to readLockA
    readLockA.lock();
    writeLockA.unlock();

    lockB.lock(); // readLockA -> lockB
    readLockA.unlock();

    // lockB -> writeLockA should fail

        "ReadWriteC -> ReadWriteA",
        "ReadWriteB -> ReadWriteC",
        "ReadWriteA -> ReadWriteB");
  }

  public void testWriteToReadLockDowngrading() {
    writeLockA.lock(); // writeLockA downgrades to readLockA
    readLockA.lock();
    writeLockA.unlock();

    lockB.lock(); // readLockA -> lockB
    readLockA.unlock();

    // lockB -> writeLockA should fail

				//
				// Go 1.21 also introduced the feature of allowing //go:build lines
				// to “downgrade” (cmp < 0) the Go version in a given file.
				// That can't be done compatibly in general, since before the
				// build lines were ignored and code got the module's Go version.
				// To work around this, downgrades are only allowed when the
				// module's Go version is Go 1.21 or later.
				//

				//
				// Go 1.21 also introduced the feature of allowing //go:build lines
				// to “downgrade” (cmp < 0) the Go version in a given file.
				// That can't be done compatibly in general, since before the
				// build lines were ignored and code got the module's Go version.
				// To work around this, downgrades are only allowed when the
				// module's Go version is Go 1.21 or later.
				//

	// Current version being written.
	xlVersionCurrent [4]byte
)

const (
	// Breaking changes.
	// Newer versions cannot be read by older software.
	// This will prevent downgrades to incompatible versions.
	xlVersionMajor = 1

	// Non breaking changes.
	// Bumping this is informational, but should be done
	// if any change is made to the data stored, bumping this

// "upgrade" or "patch", current will be returned if it is a newer
// semantic version or a chronologically later pseudo-version than the
// version that would otherwise be chosen. This prevents accidental downgrades
// from newer pre-release or development versions.
//
// The allowed function (which may be nil) is used to filter out unsuitable
// versions (see AllowedFunc documentation for details). If the query refers to