Test whether version.txt contains semantic version

semver.Compare considers all non-semantic versions as older; the root
cause of #867.

internal/cmd/cmd_version.go

@@ -14,12 +14,9 @@ var CmdVersion = &Cmd{
 	Args:    []string{"[VERSION_AT_LEAST]"},
 	Aliases: []string{"--version"},
 	Action: actionSimple(func(env Env, args []string) error {
-		semVersion := "v" + version
+		semVersion := ensureVPrefixed(version)
 		if len(args) > 1 {
-			atLeast := args[1]
-			if !strings.HasPrefix(atLeast, "v") {
-				atLeast = "v" + atLeast
-			}
+			atLeast := ensureVPrefixed(args[1])
 			if !semver.IsValid(atLeast) {
 				return fmt.Errorf("%s is not a valid semver version", atLeast)
 			}
@@ -33,3 +30,10 @@ var CmdVersion = &Cmd{
 		return nil
 	}),
 }
+
+func ensureVPrefixed(version string) string {
+	if !strings.HasPrefix(version, "v") {
+		return "v" + version
+	}
+	return version
+}

internal/cmd/cmd_version_test.go

@@ -0,0 +1,15 @@
+package cmd
+
+import (
+	"golang.org/x/mod/semver"
+	"io/ioutil"
+	"testing"
+)
+
+func TestVersionDotTxt(t *testing.T) {
+	version, _ := ioutil.ReadFile("../../version.txt")
+
+	if !semver.IsValid(ensureVPrefixed(string(version))) {
+		t.Fatalf(`version.txt does not contain a valid semantic version: %q`, version)
+	}
+}

version.txt

@@ -1 +1 @@
-2.30.2
+2.30.2