terraform: counts can't be computed

terraform/context_test.go

@@ -2428,6 +2428,23 @@ func TestContextPlan_count(t *testing.T) {
 	}
 }
 
+func TestContextPlan_countComputed(t *testing.T) {
+	m := testModule(t, "plan-count-computed")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	})
+
+	_, err := ctx.Plan(nil)
+	if err == nil {
+		t.Fatal("should error")
+	}
+}
+
 func TestContextPlan_countVar(t *testing.T) {
 	m := testModule(t, "plan-count-var")
 	p := testProvider("aws")

terraform/graph.go

@@ -1595,7 +1595,15 @@ func (p *graphSharedProvider) MergeConfig(
 
 // Expand will expand this node into a subgraph if Expand is set.
 func (n *GraphNodeResource) Expand() ([]*depgraph.Noun, error) {
-	// Expand the count out, which should be interpolated at this point
+	// If the count configuration is empty then it means that the
+	// count is computed. In this case, we set the count to one
+	// but set a flag telling upstream that we're computing.
+	if len(n.Config.RawConfig.Config()) == 0 {
+		return nil, fmt.Errorf(
+			"%s: computed count attribute not allowed", n.Resource.Id)
+	}
+
+	// Expand the count out, which should be interpolated at this point.
 	count, err := n.Config.Count()
 	if err != nil {
 		return nil, err

terraform/test-fixtures/plan-count-computed/main.tf

@@ -0,0 +1,8 @@
+resource "aws_instance" "foo" {
+    num = "2"
+    compute = "foo"
+}
+
+resource "aws_instance" "bar" {
+    count = "${aws_instance.foo.foo}"
+}