config: count can't interpolate count variables

config/config.go

@@ -258,6 +258,11 @@ func (c *Config) Validate() error {
 		// Verify count variables
 		for _, v := range r.RawCount.Variables {
 			switch v.(type) {
+			case *CountVariable:
+				errs = append(errs, fmt.Errorf(
+					"%s: resource count can't reference count variable: %s",
+					n,
+					v.FullKey()))
 			case *ModuleVariable:
 				errs = append(errs, fmt.Errorf(
 					"%s: resource count can't reference module variable: %s",

config/config_test.go

@@ -60,6 +60,13 @@ func TestConfigValidate_countInt(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_countCountVar(t *testing.T) {
+	c := testConfig(t, "validate-count-count-var")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_countModuleVar(t *testing.T) {
 	c := testConfig(t, "validate-count-module-var")
 	if err := c.Validate(); err == nil {

config/test-fixtures/validate-count-count-var/main.tf

@@ -0,0 +1,3 @@
+resource "aws_instance" "web" {
+    count = "${count.index}"
+}