improve errors for checking out tags

go/libraries/doltcore/env/actions/branch.go

@@ -365,14 +365,15 @@ func createBranch(ctx context.Context, dbData env.DbData, newBranch, startingPoi
 var emptyHash = hash.Hash{}
 
 func IsBranch(ctx context.Context, ddb *doltdb.DoltDB, str string) (bool, error) {
-	return IsBranchOnDB(ctx, ddb, str)
-}
-
-func IsBranchOnDB(ctx context.Context, ddb *doltdb.DoltDB, str string) (bool, error) {
 	dref := ref.NewBranchRef(str)
 	return ddb.HasRef(ctx, dref)
 }
 
+func IsTag(ctx context.Context, ddb *doltdb.DoltDB, str string) (bool, error) {
+	tRef := ref.NewTagRef(str)
+	return ddb.HasRef(ctx, tRef)
+}
+
 func MaybeGetCommit(ctx context.Context, dEnv *env.DoltEnv, str string) (*doltdb.Commit, error) {
 	cs, err := doltdb.NewCommitSpec(str)
 

go/libraries/doltcore/env/actions/workspace.go

@@ -36,7 +36,7 @@ func CreateWorkspace(ctx context.Context, dEnv *env.DoltEnv, name, startPoint st
 }
 
 func CreateWorkspaceOnDB(ctx context.Context, ddb *doltdb.DoltDB, name, startPoint string, headRef ref.DoltRef) error {
-	isBranch, err := IsBranchOnDB(ctx, ddb, name)
+	isBranch, err := IsBranch(ctx, ddb, name)
 	if err != nil {
 		return err
 	}

go/libraries/doltcore/sqle/dprocedures/dolt_checkout.go

@@ -163,6 +163,12 @@ func doDoltCheckout(ctx *sql.Context, args []string) (statusCode int, successMes
 		return 0, generateSuccessMessage(branchName, ""), nil
 	}
 
+	if isTag, err := actions.IsTag(ctx, dbData.Ddb, branchName); err != nil {
+		return 1, "", err
+	} else if isTag {
+		return 1, "", fmt.Errorf("error: tag '%s' is not a branch", branchName)
+	}
+
 	roots, ok := dSess.GetRoots(ctx, currentDbName)
 	if !ok {
 		return 1, "", fmt.Errorf("Could not load database %s", currentDbName)
@@ -290,11 +296,15 @@ func checkoutRemoteBranch(ctx *sql.Context, dSess *dsess.DoltSession, dbName str
 	}
 
 	if len(remoteRefs) == 0 {
+		if isTag, err := actions.IsTag(ctx, dbData.Ddb, branchName); err != nil {
+			return "", err
+		} else if isTag && apr.Contains(cli.MoveFlag) {
+			return "", fmt.Errorf(`dolt does not support a detached head state. To checkout a tag, run: 
+	dolt checkout %s -b {new_branch_namee}`, branchName)
+		}
 		if doltdb.IsValidCommitHash(branchName) && apr.Contains(cli.MoveFlag) {
-
 			// User tried to enter a detached head state, which we don't support.
 			// Inform and suggest that they check-out a new branch at this commit instead.
-
 			return "", fmt.Errorf(`dolt does not support a detached head state. To create a branch at this commit instead, run:
 
 	dolt checkout %s -b {new_branch_name}