terraform: Add test case reproducing #7241

The reproduction of issue #7421 involves a list of maps being passed to
a module, where one or more of the maps has a value which is computed
(for example, from another resource). There is a failure at the point of
use (via lookup interpolation) of the computed value of the form:

```
lookup: lookup failed to find 'elb' in:
${lookup(var.services[count.index], "elb")}
```

Where 'elb' is the key of the map.

terraform/context_plan_test.go

@@ -2325,3 +2325,43 @@ func TestContext2Plan_moduleMapLiteral(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	}
 }
+
+func TestContext2Plan_computedValueInMap(t *testing.T) {
+	m := testModule(t, "plan-computed-value-in-map")
+	p := testProvider("aws")
+	p.DiffFn = func(info *InstanceInfo, state *InstanceState, c *ResourceConfig) (*InstanceDiff, error) {
+		switch info.Type {
+		case "aws_computed_source":
+			return &InstanceDiff{
+				Attributes: map[string]*ResourceAttrDiff{
+					"computed_read_only": &ResourceAttrDiff{
+						NewComputed: true,
+					},
+				},
+			}, nil
+		}
+
+		return testDiffFn(info, state, c)
+	}
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	})
+
+	if _, err := ctx.Plan(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(plan.String())
+	expected := strings.TrimSpace(testTerraformPlanComputedValueInMap)
+	if actual != expected {
+		t.Fatalf("bad:\n%s\n\nexpected\n\n%s", actual, expected)
+	}
+}

terraform/terraform_test.go

@@ -1355,3 +1355,19 @@ aws_instance.foo:
   ID = bar
   ami = ami-abcd1234
 `
+
+const testTerraformPlanComputedValueInMap = `
+DIFF:
+
+CREATE: aws_computed_source.intermediates
+  computed_read_only: "" => "<computed>"
+
+module.test_mod:
+  CREATE: aws_instance.inner2
+    looked_up: "" => "<computed>"
+    type:      "" => "aws_instance"
+
+STATE:
+
+<no state>
+`

terraform/test-fixtures/plan-computed-value-in-map/main.tf

@@ -0,0 +1,15 @@
+resource "aws_computed_source" "intermediates" {}
+
+module "test_mod" {
+  source = "./mod"
+
+  services {
+    "exists" = "true"
+    "elb" = "${aws_computed_source.intermediates.computed_read_only}"
+  }
+
+  services {
+    "otherexists" = " true"
+    "elb" = "${aws_computed_source.intermediates.computed_read_only}"
+  }
+}

terraform/test-fixtures/plan-computed-value-in-map/mod/main.tf

@@ -0,0 +1,8 @@
+variable "services" {
+  type = "list"
+}
+
+resource "aws_instance" "inner2" {
+  looked_up = "${lookup(var.services[0], "elb")}"
+}
+