Inheritance breaks configuration file overlays

[danielyli]

Typesafe config version: 1.4.3

Summary

When a configuration object uses inheritance, non-optional environment variables are searched for before further file overlays are applied, which breaks file overlays.

Reproduction Steps

Create the following two configuration files:

File: src/main/resources/application.conf

base = {}
child = ${base} {
  foo = ${FOOBAR}
}

File: src/test/resources/application.conf

child.foo = "test-foo"

When attempting to run sbt test, the following error will appear:

Caused by: sbt.ForkMain$ForkError: com.typesafe.config.ConfigException$UnresolvedSubstitution:
application.conf @ file:/[...]/target/scala-2.13/classes/application.conf: 248: Could not resolve
substitution to a value: ${FOOBAR}

In contrast, if we change src/main/resources/application.conf to be the following, the error does not appear and child.foo is correctly set to test-foo:

child = {
  foo = ${FOOBAR}
}

Expected Behavior

The configuration value for child.foo should be test-foo for both cases.

Actual Behavior

Inheritance breaks the first case.