Validate template settings when T2 Unlimited has been enabled

* T2 Unlimited cannot be used with anything other than T2 instance types
* T2 Unlimited cannot be used with Spot Instances

builder/amazon/common/run_config.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"os"
 	"regexp"
+	"strings"
 	"time"
 
 	"github.com/hashicorp/packer/common/uuid"
@@ -142,6 +143,18 @@ func (c *RunConfig) Prepare(ctx *interpolate.Context) []error {
 		errs = append(errs, fmt.Errorf("shutdown_behavior only accepts 'stop' or 'terminate' values."))
 	}
 
+	if c.EnableT2Unlimited {
+		if c.SpotPrice != "" {
+			errs = append(errs, fmt.Errorf("Error: T2 Unlimited cannot be used in conjuction with Spot Instances"))
+		}
+		firstDotIndex := strings.Index(c.InstanceType, ".")
+		if firstDotIndex == -1 {
+			errs = append(errs, fmt.Errorf("Error determining main Instance Type from: %s", c.InstanceType))
+		} else if c.InstanceType[0:firstDotIndex] != "t2" {
+			errs = append(errs, fmt.Errorf("Error: T2 Unlimited enabled with a non-T2 Instance Type: %s", c.InstanceType))
+		}
+	}
+
 	return errs
 }