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diff --git a/src/internal/gover/gover.go b/src/internal/gover/gover.go
new file mode 100644
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--- /dev/null
+++ b/src/internal/gover/gover.go
@@ -0,0 +1,223 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package gover implements support for Go toolchain versions like 1.21.0 and 1.21rc1.
+// (For historical reasons, Go does not use semver for its toolchains.)
+// This package provides the same basic analysis that golang.org/x/mod/semver does for semver.
+//
+// The go/version package should be imported instead of this one when possible.
+// Note that this package works on "1.21" while go/version works on "go1.21".
+package gover
+
+import (
+	"cmp"
+)
+
+// A Version is a parsed Go version: major[.Minor[.Patch]][kind[pre]]
+// The numbers are the original decimal strings to avoid integer overflows
+// and since there is very little actual math. (Probably overflow doesn't matter in practice,
+// but at the time this code was written, there was an existing test that used
+// go1.99999999999, which does not fit in an int on 32-bit platforms.
+// The "big decimal" representation avoids the problem entirely.)
+type Version struct {
+	Major string // decimal
+	Minor string // decimal or ""
+	Patch string // decimal or ""
+	Kind  string // "", "alpha", "beta", "rc"
+	Pre   string // decimal or ""
+}
+
+// Compare returns -1, 0, or +1 depending on whether
+// x < y, x == y, or x > y, interpreted as toolchain versions.
+// The versions x and y must not begin with a "go" prefix: just "1.21" not "go1.21".
+// Malformed versions compare less than well-formed versions and equal to each other.
+// The language version "1.21" compares less than the release candidate and eventual releases "1.21rc1" and "1.21.0".
+func Compare(x, y string) int {
+	vx := Parse(x)
+	vy := Parse(y)
+
+	if c := CmpInt(vx.Major, vy.Major); c != 0 {
+		return c
+	}
+	if c := CmpInt(vx.Minor, vy.Minor); c != 0 {
+		return c
+	}
+	if c := CmpInt(vx.Patch, vy.Patch); c != 0 {
+		return c
+	}
+	if c := cmp.Compare(vx.Kind, vy.Kind); c != 0 { // "" < alpha < beta < rc
+		return c
+	}
+	if c := CmpInt(vx.Pre, vy.Pre); c != 0 {
+		return c
+	}
+	return 0
+}
+
+// Max returns the maximum of x and y interpreted as toolchain versions,
+// compared using Compare.
+// If x and y compare equal, Max returns x.
+func Max(x, y string) string {
+	if Compare(x, y) < 0 {
+		return y
+	}
+	return x
+}
+
+// IsLang reports whether v denotes the overall Go language version
+// and not a specific release. Starting with the Go 1.21 release, "1.x" denotes
+// the overall language version; the first release is "1.x.0".
+// The distinction is important because the relative ordering is
+//
+//	1.21 < 1.21rc1 < 1.21.0
+//
+// meaning that Go 1.21rc1 and Go 1.21.0 will both handle go.mod files that
+// say "go 1.21", but Go 1.21rc1 will not handle files that say "go 1.21.0".
+func IsLang(x string) bool {
+	v := Parse(x)
+	return v != Version{} && v.Patch == "" && v.Kind == "" && v.Pre == ""
+}
+
+// Lang returns the Go language version. For example, Lang("1.2.3") == "1.2".
+func Lang(x string) string {
+	v := Parse(x)
+	if v.Minor == "" || v.Major == "1" && v.Minor == "0" {
+		return v.Major
+	}
+	return v.Major + "." + v.Minor
+}
+
+// IsValid reports whether the version x is valid.
+func IsValid(x string) bool {
+	return Parse(x) != Version{}
+}
+
+// Parse parses the Go version string x into a version.
+// It returns the zero version if x is malformed.
+func Parse(x string) Version {
+	var v Version
+
+	// Parse major version.
+	var ok bool
+	v.Major, x, ok = cutInt(x)
+	if !ok {
+		return Version{}
+	}
+	if x == "" {
+		// Interpret "1" as "1.0.0".
+		v.Minor = "0"
+		v.Patch = "0"
+		return v
+	}
+
+	// Parse . before minor version.
+	if x[0] != '.' {
+		return Version{}
+	}
+
+	// Parse minor version.
+	v.Minor, x, ok = cutInt(x[1:])
+	if !ok {
+		return Version{}
+	}
+	if x == "" {
+		// Patch missing is same as "0" for older versions.
+		// Starting in Go 1.21, patch missing is different from explicit .0.
+		if CmpInt(v.Minor, "21") < 0 {
+			v.Patch = "0"
+		}
+		return v
+	}
+
+	// Parse patch if present.
+	if x[0] == '.' {
+		v.Patch, x, ok = cutInt(x[1:])
+		if !ok || x != "" {
+			// Note that we are disallowing prereleases (alpha, beta, rc) for patch releases here (x != "").
+			// Allowing them would be a bit confusing because we already have:
+			//	1.21 < 1.21rc1
+			// But a prerelease of a patch would have the opposite effect:
+			//	1.21.3rc1 < 1.21.3
+			// We've never needed them before, so let's not start now.
+			return Version{}
+		}
+		return v
+	}
+
+	// Parse prerelease.
+	i := 0
+	for i < len(x) && (x[i] < '0' || '9' < x[i]) {
+		if x[i] < 'a' || 'z' < x[i] {
+			return Version{}
+		}
+		i++
+	}
+	if i == 0 {
+		return Version{}
+	}
+	v.Kind, x = x[:i], x[i:]
+	if x == "" {
+		return v
+	}
+	v.Pre, x, ok = cutInt(x)
+	if !ok || x != "" {
+		return Version{}
+	}
+
+	return v
+}
+
+// cutInt scans the leading decimal number at the start of x to an integer
+// and returns that value and the rest of the string.
+func cutInt(x string) (n, rest string, ok bool) {
+	i := 0
+	for i < len(x) && '0' <= x[i] && x[i] <= '9' {
+		i++
+	}
+	if i == 0 || x[0] == '0' && i != 1 { // no digits or unnecessary leading zero
+		return "", "", false
+	}
+	return x[:i], x[i:], true
+}
+
+// CmpInt returns cmp.Compare(x, y) interpreting x and y as decimal numbers.
+// (Copied from golang.org/x/mod/semver's compareInt.)
+func CmpInt(x, y string) int {
+	if x == y {
+		return 0
+	}
+	if len(x) < len(y) {
+		return -1
+	}
+	if len(x) > len(y) {
+		return +1
+	}
+	if x < y {
+		return -1
+	} else {
+		return +1
+	}
+}
+
+// DecInt returns the decimal string decremented by 1, or the empty string
+// if the decimal is all zeroes.
+// (Copied from golang.org/x/mod/module's decDecimal.)
+func DecInt(decimal string) string {
+	// Scan right to left turning 0s to 9s until you find a digit to decrement.
+	digits := []byte(decimal)
+	i := len(digits) - 1
+	for ; i >= 0 && digits[i] == '0'; i-- {
+		digits[i] = '9'
+	}
+	if i < 0 {
+		// decimal is all zeros
+		return ""
+	}
+	if i == 0 && digits[i] == '1' && len(digits) > 1 {
+		digits = digits[1:]
+	} else {
+		digits[i]--
+	}
+	return string(digits)
+}