add failing test for invalid output with targets

Outputs that are missing references aren't always removed from the
graph, due to being filtered before their dependents are removed.

terraform/context_plan_test.go

@@ -2969,6 +2969,28 @@ STATE:
 	}
 }
 
+// ensure that outputs missing references due to targetting are removed from
+// the graph.
+func TestContext2Plan_outputContainsTargetedResource(t *testing.T) {
+	m := testModule(t, "plan-untargeted-resource-output")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		ProviderResolver: ResourceProviderResolverFixed(
+			map[string]ResourceProviderFactory{
+				"aws": testProviderFuncFixed(p),
+			},
+		),
+		Targets: []string{"module.mod.aws_instance.a"},
+	})
+
+	_, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+}
+
 // https://github.com/hashicorp/terraform/issues/4515
 func TestContext2Plan_targetedOverTen(t *testing.T) {
 	m := testModule(t, "plan-targeted-over-ten")

terraform/test-fixtures/plan-untargeted-resource-output/main.tf

@@ -0,0 +1,8 @@
+module "mod" {
+  source = "./mod"
+}
+
+
+resource "aws_instance" "c" {
+  name = "${module.mod.output}"
+}

terraform/test-fixtures/plan-untargeted-resource-output/mod/main.tf

@@ -0,0 +1,15 @@
+locals {
+  "one" = 1
+}
+
+resource "aws_instance" "a" {
+  count = "${local.one}"
+}
+
+resource "aws_instance" "b" {
+  count = "${local.one}"
+}
+
+output "output" {
+  value = "${join("", coalescelist(aws_instance.a.*.id, aws_instance.b.*.id))}"
+}