This is a Bug Report

Description

After updating to serverless 1.37 our serverless.yml (almost) silently stopped setting environment to lambdas. The issue ended up being related to default values of variables when including external files.

Here is an example config:

service: my-service

custom:
  config: ${file(config/${opt:stage, 'development'}.yml)}

provider:
  name: aws
  stage: ${self:custom.stage}
  environment: ${self:custom.config.environment}

The output of sls print -s production is:

service: my-service
custom:
  config:
    environment:
      IS_PRODUCTION: 'True'
provider:
  stage: production
  name: aws

Additionally, serverless shows a warning: A valid service attribute to satisfy the declaration 'self:custom.config.environment' could not be found.

If I remove the default value for opt:stage and use the following config, the problem disappears:

sls print -s production

service: my-service
custom:
  config:
    environment:
      IS_PRODUCTION: 'True'
provider:
  stage: production
  name: aws
  environment: <<< ENVIRONMENT IS PRESENT NOW
     $ref: '$["custom"]["config"]["environment"]'

The same config (with the default value) works just fine with serverless@1.36.3.