fix(cfn-include): NestedStack's Parameters are not converted to strings

When we include a NestedStack when creating a CfnInclude instance,
the conversion of the Parameters property of the AWS::CloudFormation::Stack resource is performed manually
(because of the eventuality that one of those Parameters was also requested to be removed when including the nested stack).
In that manual conversion, we did not correctly convert the values to strings,
which is what the underlying CfnStack class expects.
And so, if the parent stack passed a non-string primitive value to a nested stack Parameter,
like a number or boolean,
including the nested stack would fail with a validation exception.

Fixes aws#15092

packages/@aws-cdk/cloudformation-include/lib/cfn-include.ts

@@ -694,7 +694,7 @@ export class CfnInclude extends core.CfnElement {
     return nestedStackResource;
   }
 
-  private parametersForNestedStack(parameters: any, nestedStackId: string): { [key: string]: any } | undefined {
+  private parametersForNestedStack(parameters: any, nestedStackId: string): { [key: string]: string } | undefined {
     if (parameters == null) {
       return undefined;
     }
@@ -703,7 +703,7 @@ export class CfnInclude extends core.CfnElement {
     const ret: { [key: string]: string } = {};
     for (const paramName of Object.keys(parameters)) {
       if (!(paramName in parametersToReplace)) {
-        ret[paramName] = parameters[paramName];
+        ret[paramName] = cfn_parse.FromCloudFormation.getString(parameters[paramName]).value;
       }
     }
     return ret;

packages/@aws-cdk/cloudformation-include/test/nested-stacks.test.ts

@@ -334,6 +334,28 @@ describe('CDK Include for nested stacks', () => {
     }).toThrow(/Nested Stack 'AnotherChildStack' was not included in the parent template/);
   });
 
+  test('correctly handles references in nested stacks Parameters', () => {
+    new inc.CfnInclude(stack, 'ParentStack', {
+      templateFile: testTemplateFilePath('cross-stack-refs.json'),
+      loadNestedStacks: {
+        'ChildStack': {
+          templateFile: testTemplateFilePath('child-import-stack.json'),
+        },
+      },
+    });
+
+    expect(stack).toHaveResourceLike('AWS::CloudFormation::Stack', {
+      "Parameters": {
+        "Param1": {
+          "Ref": "Param",
+        },
+        "Param2": {
+          "Fn::GetAtt": ["Bucket", "Arn"],
+        },
+      },
+    });
+  });
+
   test('correctly handles renaming of references across nested stacks', () => {
     const parentTemplate = new inc.CfnInclude(stack, 'ParentStack', {
       templateFile: testTemplateFilePath('cross-stack-refs.json'),
@@ -424,6 +446,23 @@ describe('CDK Include for nested stacks', () => {
     });
   });
 
+  test('can ingest a NestedStack with a Number CFN Parameter passed as a number', () => {
+    new inc.CfnInclude(stack, 'MyScope', {
+      templateFile: testTemplateFilePath('parent-number-in-child-params.yaml'),
+      loadNestedStacks: {
+        'NestedStack': {
+          templateFile: testTemplateFilePath('child-with-number-parameter.yaml'),
+        },
+      },
+    });
+
+    expect(stack).toHaveResourceLike('AWS::CloudFormation::Stack', {
+      "Parameters": {
+        "Number": "60",
+      },
+    });
+  });
+
   test('can lazily include a single child nested stack', () => {
     const parentTemplate = new inc.CfnInclude(stack, 'ParentStack', {
       templateFile: testTemplateFilePath('parent-one-child.json'),

packages/@aws-cdk/cloudformation-include/test/test-templates/nested/child-with-number-parameter.yaml

@@ -0,0 +1,9 @@
+AWSTemplateFormatVersion: '2010-09-09'
+Parameters:
+  Number:
+    Type: Number
+Resources:
+  S3Bucket:
+    Type: AWS::S3::Bucket
+    Properties:
+      BucketName: 'testbucket1234unique'

packages/@aws-cdk/cloudformation-include/test/test-templates/nested/parent-number-in-child-params.yaml

@@ -0,0 +1,8 @@
+AWSTemplateFormatVersion: '2010-09-09'
+Resources:
+  NestedStack:
+    Type: AWS::CloudFormation::Stack
+    Properties:
+      TemplateURL: 'https://s3.amazonaws.com/masonme-cdk-test/templates/nested-bucket.yaml'
+      Parameters:
+        Number: 60