ResourceConfig.get should never return (nil, true)

Fixes a case where ResourceConfig.get inadvertently returns a nil value.

Add an integration test where assigning a map to a list via
interpolation would panic.

builtin/providers/test/resource_test.go

@@ -393,6 +393,40 @@ resource "test_resource" "foo" {
 	})
 }
 
+// Reproduces plan-time panic when the wrong type is interpolated in a list of
+// maps.
+// TODO: this should return a type error, rather than silently setting an empty
+//       list
+func TestResource_dataSourceListMapPanic(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+  required = "val"
+  required_map = {x = "y"}
+  list_of_map = "${var.maplist}"
+}
+
+variable "maplist" {
+  type = "list"
+
+  default = [
+    {a = "b"}
+  ]
+}
+				`),
+				ExpectError: nil,
+				Check: func(s *terraform.State) error {
+					return nil
+				},
+			},
+		},
+	})
+}
+
 func testAccCheckResourceDestroy(s *terraform.State) error {
 	return nil
 }

terraform/resource.go

@@ -316,7 +316,8 @@ func (c *ResourceConfig) get(
 			// prefix so were split as path components above.
 			actualKey := strings.Join(parts[i-1:], ".")
 			if prevMap, ok := previous.(map[string]interface{}); ok {
-				return prevMap[actualKey], true
+				v, ok := prevMap[actualKey]
+				return v, ok
 			}
 
 			return nil, false