config: validate that count vars are valid types

config/config.go

@@ -199,6 +199,23 @@ func (c *Config) Validate() error {
 		}
 	}
 
+	// Check that all count variables are valid.
+	for source, vs := range vars {
+		for _, v := range vs {
+			cv, ok := v.(*CountVariable)
+			if !ok {
+				continue
+			}
+
+			if cv.Type == CountValueInvalid {
+				errs = append(errs, fmt.Errorf(
+					"%s: invalid count variable: %s",
+					source,
+					cv.FullKey()))
+			}
+		}
+	}
+
 	// Check that all references to modules are valid
 	modules := make(map[string]*Module)
 	dupped := make(map[string]struct{})

config/config_test.go

@@ -95,6 +95,13 @@ func TestConfigValidate_countUserVar(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_countVarInvalid(t *testing.T) {
+	c := testConfig(t, "validate-count-var-invalid")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_dupModule(t *testing.T) {
 	c := testConfig(t, "validate-dup-module")
 	if err := c.Validate(); err == nil {

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

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