Observation

When using Java 11 APIs like java.time.Instant Android Studio will issue a warning as soon as the min API level is below the introduction of said API into the Android ecosystem. It is well known that this can be solved by using coreLibraryDesugaringEnabled and referencing the wanted desugaring library.

Now, when the said API call (e.g. java.time.Instant.now()) is used from within a library module and this library modules is imported into the main app module, we first get the same warning, if we do not specify the coreLibraryDesugaringEnabled within the library's build.gradle. We again can get rid of the warning by adding coreLibraryDesugaringEnabled to the build.gradle file (as well as referencing the wanted desugaring library).

By inspection of the resulting binary, we see that the desugared libraries are not included in the final APK (and hence the app crashes on Android versions lower than the introduction API level).

Adding the coreLibraryDesugaringEnabled flag to the app module as well, solves the issue, and the desugared libraries are present in the resulting APK.

Expectation

Either we would expect Android Studio to issue a warning, either in the build.gradle (that the flag does not have any impact) or in the library IDE, or else that the desugared libraries are added to the final APK.

Example

Attached is the most basic example (blank activity template + new Android library module)