Core: documented keep rules do not preserve reflective model construction under strict R8 #2193

JSON parsing can fail under strict R8 full mode even when using the library’s documented ProGuard rules. I reproduced this with the latest published google-http-client and google-http-client-gson 2.2.0.

The documented rules preserve generic signatures, annotations, and @Key fields, but do not explicitly preserve model classes and their public no-argument constructors. In the reproducer, R8 makes a GenericJson response model abstract and removes its constructor. Parsing then fails when the library attempts to construct that model reflectively.

Environment

• macOS 26.5
• OpenJDK/JBR 25.0.3
• google-http-client and google-http-client-gson 2.2.0
• Gson 2.11.0 and Guava 33.4.8-android
• R8 9.4.24, bundled with AGP 9.4.1
• R8 release mode, with setProguardCompatibility(false) and enableLegacyFullModeForKeepRules(false)

Reproduction

The attached reproducer.zip contains synthetic model fixtures, both rule configurations, and a runner. Extract it and run:

cd reproducer
python3 run.py --r8-jar /path/to/builder-9.4.1.jar

The runner downloads the pinned parsing dependencies, compiles an unminified control, and runs R8 with the documented rules and with the workaround below. R8 processes the fixture and both Google HTTP Client JARs together. Gson and Guava are supplied as library dependencies.

The runner checks these results automatically:

The parsing tests assert scalar values and nested model types. The runner exits successfully only when the expected failures and successes are observed.

Representative failure

The retraced GenericJson failure includes:

Caused by: java.lang.IllegalArgumentException: unable to create new instance of class repro.a because it is abstract and because it has no accessible default constructor
    at com.google.api.client.util.Types.handleExceptionForNewInstance(Types.java:162)
    at com.google.api.client.util.Types.newInstance(Types.java:117)
    at com.google.api.client.util.Data.newMapInstance(Data.java:550)
    at com.google.api.client.json.JsonParser.parseValue(JsonParser.java:766)
...
Caused by: java.lang.NoSuchMethodException: repro.a.<init>()

The mapping identifies repro.a as the fixture’s GenericRoot model. The other two parsing failures are ClassCastException failures associated with lost reflected model/type retention; they are not separate demonstrations of missing constructors.

Workaround

Keep the existing attribute and annotated-field rules, and additionally use:

-keepclasseswithmembers,allowobfuscation class * {
    @com.google.api.client.util.Key <fields>;
    public <init>();
}

This fixes all three parsing cases in the reproducer.

The original annotated-field rule remains necessary for models that do not match this constructor rule, including request classes with only parameterized constructors. It also preserves field names when @Key uses its default name, while the additional rule permits class-name obfuscation.

Reflective construction in 2.2.0

In 2.2.0, JSON model construction reaches Types.newInstance(), which calls Class.newInstance() and requires a public no-argument constructor.

The release’s standalone ProGuard configuration preserves attributes and annotated fields but omits constructor protection. The core JAR also does not bundle Android consumer keep rules.

Could the project update the documented rules, ship appropriate consumer rules, and add an R8 regression test for reflective model construction?

Related: #1843 reports missing documentation and a similar full-mode construction failure. PR #2171 restores the documentation using the existing rules.

For simplicity, this reproducer uses JVM classfile output. It does not test an Android APK using 2.2.0.