core: Add context test for empty lists as module outputs

This test illustrates a failure which occurs during the Input walk, if
an interpolation is used with the input of a splat operation resulting
in a multi-variable.

The bug was found during use of the RC2, but does not correspond to an
open issue at present.

terraform/context_input_test.go

@@ -49,6 +49,27 @@ func TestContext2Input(t *testing.T) {
 	}
 }
 
+func TestContext2Input_moduleComputedOutputElement(t *testing.T) {
+	m := testModule(t, "input-module-computed-output-element")
+	p := testProvider("aws")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	})
+
+	p.InputFn = func(i UIInput, c *ResourceConfig) (*ResourceConfig, error) {
+		return c, nil
+	}
+
+	if err := ctx.Input(InputModeStd); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+}
+
 func TestContext2Input_badVarDefault(t *testing.T) {
 	m := testModule(t, "input-bad-var-default")
 	p := testProvider("aws")

terraform/test-fixtures/input-module-computed-output-element/main.tf

@@ -0,0 +1,9 @@
+module "b" {
+  source = "./modb"
+}
+
+module "a" {
+  source = "./moda"
+
+  single_element = "${element(module.b.computed_list, 0)}"
+}

terraform/test-fixtures/input-module-computed-output-element/moda/main.tf

@@ -0,0 +1,3 @@
+variable "single_element" {
+  type = "string"
+}

terraform/test-fixtures/input-module-computed-output-element/modb/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "test" {
+  count = 3
+}
+
+output "computed_list" {
+  value = ["${aws_instance.test.*.id}"]
+}