[Hilt] Incremental kapt does not always regenerate references to other modules

This may be a duplicate of #1684 but I wanted to provide a reproducible example just in case, since that issue seems to have gone stale (and was not reproducible at the time). The sample can be found here: https://github.com/damianw/HiltIncrementality

The sample consists of two Gradle modules:

• :app: The application, which is just a single activity with a TextView which toString’s the injected dependencies. See MainActivity.kt.
• :library: A library which provides multiple implementations of CoffeeMaker using multi- bindings. See Coffee.kt.

Issue

The issue has something to do with failing to regenerate code after making changes in library. To start, build the app from clean:

./gradlew :app:assembleDebug

This should build successfully. Then, uncomment the val water: Water lines from Coffee.kt, and build :app:assembleDebug again. The expected behavior is that this builds successfully. Instead, the build will fail with:

/Users/damian/Development/HiltIncrementality/app/build/generated/source/kapt/debug/com/example/hiltincrementality/DaggerApp_HiltComponents_SingletonC.java:61: error: constructor DripCoffeeMaker in class DripCoffeeMaker cannot be applied to given types;
          local = new DripCoffeeMaker();
                  ^
  required: Water
  found: no arguments
  reason: actual and formal argument lists differ in length
/Users/damian/Development/HiltIncrementality/app/build/generated/source/kapt/debug/com/example/hiltincrementality/DaggerApp_HiltComponents_SingletonC.java:75: error: constructor PourOverCoffeeMaker in class PourOverCoffeeMaker cannot be applied to given types;
          local = new PourOverCoffeeMaker();
                  ^
  required: Water
  found: no arguments
  reason: actual and formal argument lists differ in length
/Users/damian/Development/HiltIncrementality/app/build/generated/source/kapt/debug/com/example/hiltincrementality/DaggerApp_HiltComponents_SingletonC.java:89: error: constructor EspressoCoffeeMaker in class EspressoCoffeeMaker cannot be applied to given types;
          local = new EspressoCoffeeMaker();
                  ^
  required: Water
  found: no arguments
  reason: actual and formal argument lists differ in length

Cleaning (either which Gradle or rm -rf app/build) will fix the problem (allowing the app to build again) at the loss of incrementality. The same problem now exists in reverse, if you were to re- comment the val water: Water line .

[danysantiago]

Done, created an issue with a more minimal sample project: https://youtrack.jetbrains.com/issue/KT-42182

[danysantiago]

An update: I’ve created the same project in Java (HiltIncrementalityJava.zip) and was not able to reproduce the issue, which is making kapt very suspicious as the possible culprit of this issue. 🧐