In order to keep my Lambda functions declarations DRY, I clone (using lodash) default Lambda properties and reuse them:

const defaultLayer = lambda.LayerVersion.fromLayerVersionArn(this, 'DefaultLayer', 'some arn');
const extraLayer = lambda.LayerVersion.fromLayerVersionArn(this, 'ExtraLayer', 'another arn');

const defaultLambaProps = {
  timeout: 30,
  runtime: lambda.Runtime.Python37,
  handler: 'index.handler',
  layers: [
    defaultLayer,
  ],
};

const func = new lambda.Function(this, 'Func',
  _.merge(_.cloneDeep(defaultLambaProps), {
    code: lambda.Code.asset(`funcCode`),
    layers: defaultLambaProps.layers.concat([
      extraLayer,
    ])
  })
);

However, since 0.31.0, this strategy fails:

Error: This lambda function uses a runtime that is incompatible with this layer (python3.7 is not in [nodejs, nodejs4.3, nodejs6.10, nodejs8.10, python2.7, python3.6, python3.7, java8, dotnetcore1.0, dotnetcore2.0, dotnetcore2.1, go1.x, ruby2.5, provided])
    at Function.addLayer (/Users/spgingras/entity-data-pipeline/services/cdk-app/node_modules/@aws-cdk/aws-lambda/lib/function.ts:501:13)
    at new Function (/Users/spgingras/entity-data-pipeline/services/cdk-app/node_modules/@aws-cdk/aws-lambda/lib/function.ts:445:12)
    ...

Note that the error message is confusing: python3.7 is not in [..., python3.7, ...].

I realized that after cloning occured, defaultLayer and extraLayer did not share the same Runtime.All array, resulting in layer.compatibleRuntimes.indexOf(this.runtime) returning -1.

I fixed this problem by implementing deep equality on Runtime (see Runtime.equals(other)).

Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.