fix(core): allow embedding condition expression as strings (#2007)

Expose the underlying `toString` in the `IConditionExpression` interface
so they can be embedded as string values in jsii languages.

Fixes #1984

packages/@aws-cdk/cdk/lib/cloudformation/condition.ts

@@ -74,4 +74,23 @@ export interface IConditionExpression {
    * Returns a JSON node that represents this condition expression
    */
   resolve(context: ResolveContext): any;
+
+  /**
+   * Returns a string token representation of this condition expression, which
+   * resolves to the CloudFormation condition JSON during synthesis.
+   *
+   * You can use `toString` when you wish to embed a condition expression
+   * in a property value that accepts a `string`. For example:
+   *
+   * ```ts
+   * new sqs.Queue(this, 'MyQueue', {
+   *   queueName: Fn.conditionIf('Condition', 'Hello', 'World').toString()
+   * });
+   * ```
+   *
+   * NOTE: we need this explicitly here despite the fact that in JavaScript this would
+   * "just work" since conditions are eventually tokens that implement `toString`,
+   * in order for jsii languages like Java to proxy this to jsii.
+   */
+  toString(): string;
 }

packages/@aws-cdk/cdk/test/cloudformation/test.condition.ts

@@ -28,5 +28,33 @@ export = {
           { 'Fn::Not': [ { Condition: 'Condition3' } ] } ] } } });
 
     test.done();
+  },
+
+  'condition expressions can be embedded as strings'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const propValue: string = cdk.Fn.conditionIf('Cond', 'A', 'B').toString();
+
+    // WHEN
+    new cdk.Resource(stack, 'MyResource', {
+      type: 'AWS::Foo::Bar',
+      properties: {
+        StringProp: propValue
+      }
+    });
+
+    // THEN
+    test.ok(cdk.unresolved(propValue));
+    test.deepEqual(stack.toCloudFormation(), {
+      Resources: {
+        MyResource: {
+          Type: 'AWS::Foo::Bar',
+          Properties: {
+            StringProp: { 'Fn::If': [ 'Cond', 'A', 'B' ] }
+          }
+        }
+      }
+    });
+    test.done();
   }
 };