failing test for interpolated count from plan

An interpolated count value that is determined during plan, is lost
during plan serialization, causing apply to fail when the interpolation
string can't be evaluated.

terraform/context_apply_test.go

@@ -9472,5 +9472,72 @@ func TestContext2Apply_providersFromState(t *testing.T) {
 
 		})
 	}
-
+}
+
+func TestContext2Apply_plannedInterpolatedCount(t *testing.T) {
+	m := testModule(t, "apply-interpolated-count")
+
+	p := testProvider("aws")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+
+	providerResolver := ResourceProviderResolverFixed(
+		map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	)
+
+	s := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_instance.test": {
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "foo",
+						},
+						Provider: "provider.aws",
+					},
+				},
+			},
+		},
+	}
+
+	ctx := testContext2(t, &ContextOpts{
+		Module:           m,
+		ProviderResolver: providerResolver,
+		State:            s,
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("plan failed: %s", err)
+	}
+
+	// We'll marshal and unmarshal the plan here, to ensure that we have
+	// a clean new context as would be created if we separately ran
+	// terraform plan -out=tfplan && terraform apply tfplan
+	var planBuf bytes.Buffer
+	err = WritePlan(plan, &planBuf)
+	if err != nil {
+		t.Fatalf("failed to write plan: %s", err)
+	}
+	plan, err = ReadPlan(&planBuf)
+	if err != nil {
+		t.Fatalf("failed to read plan: %s", err)
+	}
+
+	ctx, err = plan.Context(&ContextOpts{
+		ProviderResolver: providerResolver,
+	})
+	if err != nil {
+		t.Fatalf("failed to create context for plan: %s", err)
+	}
+
+	// Applying the plan should now succeed
+	_, err = ctx.Apply()
+	if err != nil {
+		t.Fatalf("apply failed: %s", err)
+	}
 }

terraform/test-fixtures/apply-interpolated-count/main.tf

@@ -0,0 +1,11 @@
+variable "instance_count" {
+  default = 1
+}
+
+resource "aws_instance" "test" {
+  count = "${var.instance_count}"
+}
+
+resource "aws_instance" "dependent" {
+  count = "${aws_instance.test.count}"
+}