Repository: google/gson (Java)
Task type: Feature

Issue:
Improve default ProGuard / R8 rules and Troubleshooting Guide
# Problem solved by the feature
Improve the default ProGuard / R8 rules in `META-INF/proguard/gson.pro` added by #2397, and the Troubleshooting Guide.

# Feature description
I was testing the new default rules with an Android app and Kotlin classes, and there might be the following areas to improve.

### Troubleshooting Guide: `JsonIOException`: 'Abstract classes can't be instantiated!' (R8)

The Troubleshooting Guide currently suggests the following:
```
# Keep the no-args constructor of the deserialized class
-keepclassmembers class com.example.MyClass {
  <init>();
}
```

This means only the no-args constructor is kept, but not any other constructors. However, especially for Kotlin I assume it is common that classes don't have a no-args constructor because you might declare properties and the primary constructor at once, e.g. `class MyClass(val s: String)` (see [Kotlin documentation](https://kotlinlang.org/docs/classes.html#constructors)). Not sure though if using that is a good idea in combination with Gson because it then requires JDK Unsafe to create instances, but that is a different topic.

So there are two questions:
- Should the Troubleshooting Guide rather recommend `<init>(...);` (all constructors), so that the users don't experience issues with R8 anymore (but implicitly depend on JDK Unsafe; using `GsonBuilder.disableJdkUnsafe()` would cause Gson to fail with a different exception)
- Should the Troubleshooting Guide recommend `-keep` instead of `-keepclassmembers`. In case the `<init>` rule for the class properly matches a constructor it might not matter. But in case `<init>();` is used, but a no-args constructor does not exist, then `-keep` seems to at least have the side-effect that R8 does not make the class abstract (and then using JDK Unsafe an instance can be created).

### Troubleshooting Guide: Recommend `@Keep` for Android
Instead of configuring ProGuard / R8 rules, it might be easier for Android developers to use `@Keep` on the corresponding class or constructor, see https://developer.android.com/studio/write/annotations#keep

### Preventing R8 from making classes abstract / removing no-args constructor

It would be good if we could adjust the default rules to avoid the "Abstract classes can't be instantiated!" exception (for most cases) in the first place.

There is probably no general way to detect if a class might be used with Gson, but the most reliable variant might be to keep the constructor if any of the Gson annotations is used by a class.

This can probably be achieved with an [`-if` rule](https://www.guardsquare.com/manual/configuration/usage#if) (see also [answers to this Stack Overflow question](https://stackoverflow.com/q/63162818)). For example:
```
# If class has fields with `@SerializedName` annotation keep its constructors
-if class * {
  @com.google.gson.annotations.SerializedName <fields>;
}
-keep class <1> {
  <init>(...);
}
```
(duplicated for all Gson annotations)

But it looks like this is not enough, maybe because there is no `-keep` (or `-keepclasseswithmembers`) rule for the class in the first place so `-if` somehow has no effect? You additionally need the following (duplicated for all Gson annotations probably):
```
-keepclasseswithmembers class * {
  @com.google.gson.annotations.SerializedName <fields>;
}
```
Only then the class is not made abstract and the constructor is properly kept...

(As side note: Having only the `-keepclasseswithmembers ... { @SerializedName }` above, but not the `-if ... -keep` seems to prevent R8 from making the class abstract, but still removes all constructors, so users would be dependent on JDK Unsafe then.)

Or similar to https://github.com/google/gson/issues/2379#issuecomment-1564517629 maybe the following would work as well:
```
-keepclasseswithmembers class * {
  <init>(...);
  @com.google.gson.annotations.SerializedName <fields>;
}
```
There is however probably no need for the `-if` rule shown in the original comment, unless that has some special effect, see https://github.com/google/gson/pull/2397#issuecomment-1575236086.

### R8 makes class abstract but still keeps constructors

It also looks like the Android app build creates (for some reason) additional constructors / keeps constructors even when the class is made abstract. Due to this, the Troubleshooting Guide URLs are unfortunately not included in the exception messages (they are only added at the moment when there are no constructors).
Not really sure why these constructors exist in the first place and are kept even when the class is made abstract. In the R8 integration test of Gson this behavior was not seen.

Maybe we should remove the `getDeclaredConstructors().length == 0` check if the class is abstract and always refer to the Troubleshooting Guide. The guide would have to be adjusted then though to also properly cover the case where the class was indeed abstract in the source code.

---

Any additional feedback is highly appreciated!



Discussion:

Solution patch:
diff --git a/Troubleshooting.md b/Troubleshooting.md
--- a/Troubleshooting.md
+++ b/Troubleshooting.md
@@ -313,6 +313,8 @@ Note: For newer Gson versions these rules might be applied automatically; make s
 
 **Symptom:** A `JsonIOException` with the message 'Abstract classes can't be instantiated!' is thrown; the class mentioned in the exception message is not actually `abstract` in your source code, and you are using the code shrinking tool R8 (Android app builds normally have this configured by default).
 
+Note: If the class which you are trying to deserialize is actually abstract, then this exception is probably unrelated to R8 and you will have to implement a custom [`InstanceCreator`](https://javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/InstanceCreator.html) or [`TypeAdapter`](https://javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/TypeAdapter.html) which creates an instance of a non-abstract subclass of the class.
+
 **Reason:** The code shrinking tool R8 performs optimizations where it removes the no-args constructor from a class and makes the class `abstract`. Due to this Gson cannot create an instance of the class.
 
 **Solution:** Make sure the class has a no-args constructor, then adjust your R8 configuration file to keep the constructor of the class. For example:
@@ -324,6 +326,10 @@ Note: For newer Gson versions these rules might be applied automatically; make s
 }
 ```
 
+You can also use `<init>(...);` to keep all constructors of that class, but then you might actually rely on `sun.misc.Unsafe` on both JDK and Android to create classes without no-args constructor, see [`GsonBuilder.disableJdkUnsafe()`](https://javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/GsonBuilder.html#disableJdkUnsafe()) for more information.
+
 For Android you can add this rule to the `proguard-rules.pro` file, see also the [Android documentation](https://developer.android.com/build/shrink-code#keep-code). In case the class name in the exception message is obfuscated, see the Android documentation about [retracing](https://developer.android.com/build/shrink-code#retracing).
 
-Note: If the class which you are trying to deserialize is actually abstract, then this exception is probably unrelated to R8 and you will have to implement a custom [`InstanceCreator`](https://javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/InstanceCreator.html) or [`TypeAdapter`](https://javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/TypeAdapter.html) which creates an instance of a non-abstract subclass of the class.
+For Android you can alternatively use the [`@Keep` annotation](https://developer.android.com/studio/write/annotations#keep) on the class or constructor you want to keep. That might be easier than having to maintain a custom R8 configuration.
+
+Note that the latest Gson versions (> 2.10.1) specify a default R8 configuration. If your class is a top-level class or is `static`, has a no-args constructor and its fields are annotated with Gson's [`@SerializedName`](https://www.javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/annotations/SerializedName.html), you might not have to perform any additional R8 configuration.
diff --git a/examples/android-proguard-example/README.md b/examples/android-proguard-example/README.md
--- a/examples/android-proguard-example/README.md
+++ b/examples/android-proguard-example/README.md
@@ -12,6 +12,9 @@ details on how ProGuard can be configured.
 The R8 code shrinker uses the same rule format as ProGuard, but there are differences between these two
 tools. Have a look at R8's Compatibility FAQ, and especially at the [Gson section](https://r8.googlesource.com/r8/+/refs/heads/main/compatibility-faq.md#gson).
 
-Note that newer Gson versions apply some of the rules shown in `proguard.cfg` automatically by default,
+Note that the latest Gson versions (> 2.10.1) apply some of the rules shown in `proguard.cfg` automatically by default,
 see the file [`gson/META-INF/proguard/gson.pro`](/gson/src/main/resources/META-INF/proguard/gson.pro) for
-the Gson version you are using.
+the Gson version you are using. In general if your classes are top-level classes or are `static`, have a no-args constructor and their fields are annotated with Gson's [`@SerializedName`](https://www.javadoc.io/doc/com.google.code.gson/gson/latest/com.google.gson/com/google/gson/annotations/SerializedName.html), you might not have to perform any additional ProGuard or R8 configuration.
+
+An alternative to writing custom keep rules for your classes in the ProGuard configuration can be to use
+Android's [`@Keep` annotation](https://developer.android.com/studio/write/annotations#keep).
diff --git a/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java b/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
--- a/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
+++ b/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
@@ -76,14 +76,15 @@ static String checkInstantiable(Class<?> c) {
     if (Modifier.isAbstract(modifiers)) {
       // R8 performs aggressive optimizations where it removes the default constructor of a class
       // and makes the class `abstract`; check for that here explicitly
-      if (c.getDeclaredConstructors().length == 0) {
-        return "Abstract classes can't be instantiated! Adjust the R8 configuration or register"
-            + " an InstanceCreator or a TypeAdapter for this type. Class name: " + c.getName()
-            + "\nSee " + TroubleshootingGuide.createUrl("r8-abstract-class");
-      }
-
-      return "Abstract classes can't be instantiated! Register an InstanceCreator"
-          + " or a TypeAdapter for this type. Class name: " + c.getName();
+      /*
+       * Note: Ideally should only show this R8-specific message when it is clear that R8 was
+       * used (e.g. when `c.getDeclaredConstructors().length == 0`), but on Android where this
+       * issue with R8 occurs most, R8 seems to keep some constructors for some reason while
+       * still making the class abstract
+       */
+      return "Abstract classes can't be instantiated! Adjust the R8 configuration or register"
+          + " an InstanceCreator or a TypeAdapter for this type. Class name: " + c.getName()
+          + "\nSee " + TroubleshootingGuide.createUrl("r8-abstract-class");
     }
     return null;
   }
diff --git a/gson/src/main/resources/META-INF/proguard/gson.pro b/gson/src/main/resources/META-INF/proguard/gson.pro
--- a/gson/src/main/resources/META-INF/proguard/gson.pro
+++ b/gson/src/main/resources/META-INF/proguard/gson.pro
@@ -13,7 +13,7 @@
 
 # Keep Gson annotations
 # Note: Cannot perform finer selection here to only cover Gson annotations, see also https://stackoverflow.com/q/47515093
--keepattributes *Annotation*
+-keepattributes RuntimeVisibleAnnotations,AnnotationDefault
 
 
 ### The following rules are needed for R8 in "full mode" which only adheres to `-keepattribtues` if
@@ -24,10 +24,10 @@
 -keep class com.google.gson.reflect.TypeToken { *; }
 
 # Keep any (anonymous) classes extending TypeToken
--keep class * extends com.google.gson.reflect.TypeToken
+-keep,allowobfuscation class * extends com.google.gson.reflect.TypeToken
 
 # Keep classes with @JsonAdapter annotation
--keep @com.google.gson.annotations.JsonAdapter class *
+-keep,allowobfuscation,allowoptimization @com.google.gson.annotations.JsonAdapter class *
 
 # Keep fields with @SerializedName annotation, but allow obfuscation of their names
 -keepclassmembers,allowobfuscation class * {
@@ -35,7 +35,9 @@
 }
 
 # Keep fields with any other Gson annotation
--keepclassmembers class * {
+# Also allow obfuscation, assuming that users will additionally use @SerializedName or
+# other means to preserve the field names
+-keepclassmembers,allowobfuscation class * {
   @com.google.gson.annotations.Expose <fields>;
   @com.google.gson.annotations.JsonAdapter <fields>;
   @com.google.gson.annotations.Since <fields>;
@@ -44,15 +46,25 @@
 
 # Keep no-args constructor of classes which can be used with @JsonAdapter
 # By default their no-args constructor is invoked to create an adapter instance
--keep class * extends com.google.gson.TypeAdapter {
+-keepclassmembers class * extends com.google.gson.TypeAdapter {
   <init>();
 }
--keep class * implements com.google.gson.TypeAdapterFactory {
+-keepclassmembers class * implements com.google.gson.TypeAdapterFactory {
   <init>();
 }
--keep class * implements com.google.gson.JsonSerializer {
+-keepclassmembers class * implements com.google.gson.JsonSerializer {
   <init>();
 }
--keep class * implements com.google.gson.JsonDeserializer {
+-keepclassmembers class * implements com.google.gson.JsonDeserializer {
   <init>();
 }
+
+# If a class is used in some way by the application, and has fields annotated with @SerializedName
+# and a no-args constructor, keep those fields and the constructor
+# Based on https://issuetracker.google.com/issues/150189783#comment11
+# See also https://github.com/google/gson/pull/2420#discussion_r1241813541 for a more detailed explanation
+-if class *
+-keepclasseswithmembers,allowobfuscation,allowoptimization class <1> {
+  <init>();
+  @com.google.gson.annotations.SerializedName <fields>;
+}
diff --git a/shrinker-test/pom.xml b/shrinker-test/pom.xml
--- a/shrinker-test/pom.xml
+++ b/shrinker-test/pom.xml
@@ -191,6 +191,8 @@
         <dependencies>
           <dependency>
             <!-- R8 dependency used above -->
+            <!-- Note: For some reason Maven shows the warning "Missing POM for com.android.tools:r8:jar",
+              but it appears that can be ignored -->
             <groupId>com.android.tools</groupId>
             <artifactId>r8</artifactId>
             <version>8.0.40</version>
diff --git a/shrinker-test/proguard.pro b/shrinker-test/proguard.pro
--- a/shrinker-test/proguard.pro
+++ b/shrinker-test/proguard.pro
@@ -18,6 +18,7 @@
 -keep class com.example.DefaultConstructorMain {
   public static java.lang.String runTest();
   public static java.lang.String runTestNoJdkUnsafe();
+  public static java.lang.String runTestNoDefaultConstructor();
 }
 
 
@@ -27,3 +28,21 @@
 -keepclassmembers class com.example.ClassWithNamedFields {
   !transient <fields>;
 }
+
+-keepclassmembernames class com.example.ClassWithExposeAnnotation {
+  <fields>;
+}
+-keepclassmembernames class com.example.ClassWithJsonAdapterAnnotation {
+  ** f;
+}
+-keepclassmembernames class com.example.ClassWithVersionAnnotations {
+  <fields>;
+}
+
+
+-keepclassmembernames class com.example.DefaultConstructorMain$TestClass {
+  <fields>;
+}
+-keepclassmembernames class com.example.DefaultConstructorMain$TestClassNotAbstract {
+  <fields>;
+}
diff --git a/shrinker-test/r8.pro b/shrinker-test/r8.pro
--- a/shrinker-test/r8.pro
+++ b/shrinker-test/r8.pro
@@ -4,20 +4,6 @@
 ### The following rules are needed for R8 in "full mode", which performs more aggressive optimizations than ProGuard
 ### See https://r8.googlesource.com/r8/+/refs/heads/main/compatibility-faq.md#r8-full-mode
 
-# Keep the no-args constructor of deserialized classes
--keepclassmembers class com.example.ClassWithDefaultConstructor {
-  <init>();
-}
--keepclassmembers class com.example.GenericClasses$GenericClass {
-  <init>();
-}
--keepclassmembers class com.example.GenericClasses$UsingGenericClass {
-  <init>();
-}
--keepclassmembers class com.example.GenericClasses$GenericUsingGenericClass {
-  <init>();
-}
-
 # For classes with generic type parameter R8 in "full mode" requires to have a keep rule to
 # preserve the generic signature
 -keep,allowshrinking,allowoptimization,allowobfuscation,allowaccessmodification class com.example.GenericClasses$GenericClass
@@ -25,12 +11,14 @@
 
 # Don't obfuscate class name, to check it in exception message
 -keep,allowshrinking,allowoptimization class com.example.DefaultConstructorMain$TestClass
+-keep,allowshrinking,allowoptimization class com.example.DefaultConstructorMain$TestClassWithoutDefaultConstructor
+
 # This rule has the side-effect that R8 still removes the no-args constructor, but does not make the class abstract
 -keep class com.example.DefaultConstructorMain$TestClassNotAbstract {
   @com.google.gson.annotations.SerializedName <fields>;
 }
 
 # Keep enum constants which are not explicitly used in code
--keep class com.example.EnumClass {
+-keepclassmembers class com.example.EnumClass {
   ** SECOND;
 }
diff --git a/shrinker-test/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java b/shrinker-test/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java
--- a/shrinker-test/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java
+++ b/shrinker-test/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java
@@ -23,6 +23,7 @@
  */
 public class ClassWithJsonAdapterAnnotation {
   // For this field don't use @SerializedName and ignore it for deserialization
+  // Has custom ProGuard rule to keep the field name
   @JsonAdapter(value = Adapter.class, nullSafe = false)
   DummyClass f;
 
diff --git a/shrinker-test/src/main/java/com/example/DefaultConstructorMain.java b/shrinker-test/src/main/java/com/example/DefaultConstructorMain.java
--- a/shrinker-test/src/main/java/com/example/DefaultConstructorMain.java
+++ b/shrinker-test/src/main/java/com/example/DefaultConstructorMain.java
@@ -8,14 +8,24 @@
 
 public class DefaultConstructorMain {
   static class TestClass {
-    @SerializedName("s")
     public String s;
   }
 
   // R8 rule for this class still removes no-args constructor, but doesn't make class abstract
   static class TestClassNotAbstract {
+    public String s;
+  }
+
+  // Current Gson ProGuard rules only keep default constructor (and only then prevent R8 from
+  // making class abstract); other constructors are ignored to suggest to user adding default
+  // constructor instead of implicitly relying on JDK Unsafe
+  static class TestClassWithoutDefaultConstructor {
     @SerializedName("s")
     public String s;
+
+    public TestClassWithoutDefaultConstructor(String s) {
+      this.s = s;
+    }
   }
 
   /**
@@ -34,4 +44,12 @@ public static String runTestNoJdkUnsafe() {
     TestClassNotAbstract deserialized = gson.fromJson("{\"s\": \"value\"}", same(TestClassNotAbstract.class));
     return deserialized.s;
   }
+
+  /**
+   * Main entrypoint, called by {@code ShrinkingIT.testNoDefaultConstructor()}.
+   */
+  public static String runTestNoDefaultConstructor() {
+    TestClassWithoutDefaultConstructor deserialized = new Gson().fromJson("{\"s\":\"value\"}", same(TestClassWithoutDefaultConstructor.class));
+    return deserialized.s;
+  }
 }


Test patch:
diff --git a/gson/src/test/java/com/google/gson/functional/Java17RecordTest.java b/gson/src/test/java/com/google/gson/functional/Java17RecordTest.java
--- a/gson/src/test/java/com/google/gson/functional/Java17RecordTest.java
+++ b/gson/src/test/java/com/google/gson/functional/Java17RecordTest.java
@@ -89,7 +89,7 @@ public int i() {
 
     var exception = assertThrows(JsonIOException.class, () -> gson.getAdapter(LocalRecord.class));
     assertThat(exception).hasMessageThat()
-            .isEqualTo("@SerializedName on method '" + LocalRecord.class.getName() + "#i()' is not supported");
+        .isEqualTo("@SerializedName on method '" + LocalRecord.class.getName() + "#i()' is not supported");
   }
 
   @Test
@@ -154,7 +154,7 @@ record LocalRecord(String s) {
     // TODO: Adjust this once Gson throws more specific exception type
     catch (RuntimeException e) {
       assertThat(e).hasMessageThat()
-              .isEqualTo("Failed to invoke constructor '" + LocalRecord.class.getName() + "(String)' with args [value]");
+          .isEqualTo("Failed to invoke constructor '" + LocalRecord.class.getName() + "(String)' with args [value]");
       assertThat(e).hasCauseThat().isSameInstanceAs(LocalRecord.thrownException);
     }
   }
@@ -227,7 +227,7 @@ public void testPrimitiveJsonNullValue() {
     String s = "{'aString': 's', 'aByte': null, 'aShort': 0}";
     var e = assertThrows(JsonParseException.class, () -> gson.fromJson(s, RecordWithPrimitives.class));
     assertThat(e).hasMessageThat()
-            .isEqualTo("null is not allowed as value for record component 'aByte' of primitive type; at path $.aByte");
+        .isEqualTo("null is not allowed as value for record component 'aByte' of primitive type; at path $.aByte");
   }
 
   /**
@@ -384,8 +384,8 @@ record Blocked(int b) {}
 
     var exception = assertThrows(JsonIOException.class, () -> gson.toJson(new Blocked(1)));
     assertThat(exception).hasMessageThat()
-            .isEqualTo("ReflectionAccessFilter does not permit using reflection for class " + Blocked.class.getName() +
-                ". Register a TypeAdapter for this type or adjust the access filter.");
+        .isEqualTo("ReflectionAccessFilter does not permit using reflection for class " + Blocked.class.getName() +
+            ". Register a TypeAdapter for this type or adjust the access filter.");
   }
 
   @Test
@@ -396,15 +396,15 @@ public void testReflectionFilterBlockInaccessible() {
 
     var exception = assertThrows(JsonIOException.class, () -> gson.toJson(new PrivateRecord(1)));
     assertThat(exception).hasMessageThat()
-            .isEqualTo("Constructor 'com.google.gson.functional.Java17RecordTest$PrivateRecord(int)' is not accessible and"
-                + " ReflectionAccessFilter does not permit making it accessible. Register a TypeAdapter for the declaring"
-                + " type, adjust the access filter or increase the visibility of the element and its declaring type.");
+        .isEqualTo("Constructor 'com.google.gson.functional.Java17RecordTest$PrivateRecord(int)' is not accessible and"
+            + " ReflectionAccessFilter does not permit making it accessible. Register a TypeAdapter for the declaring"
+            + " type, adjust the access filter or increase the visibility of the element and its declaring type.");
 
     exception = assertThrows(JsonIOException.class, () -> gson.fromJson("{}", PrivateRecord.class));
     assertThat(exception).hasMessageThat()
-            .isEqualTo("Constructor 'com.google.gson.functional.Java17RecordTest$PrivateRecord(int)' is not accessible and"
-                + " ReflectionAccessFilter does not permit making it accessible. Register a TypeAdapter for the declaring"
-                + " type, adjust the access filter or increase the visibility of the element and its declaring type.");
+        .isEqualTo("Constructor 'com.google.gson.functional.Java17RecordTest$PrivateRecord(int)' is not accessible and"
+            + " ReflectionAccessFilter does not permit making it accessible. Register a TypeAdapter for the declaring"
+            + " type, adjust the access filter or increase the visibility of the element and its declaring type.");
 
     assertThat(gson.toJson(new PublicRecord(1))).isEqualTo("{\"i\":1}");
     assertThat(gson.fromJson("{\"i\":2}", PublicRecord.class)).isEqualTo(new PublicRecord(2));
@@ -427,7 +427,8 @@ record LocalRecord(int i) {}
 
     var exception = assertThrows(JsonIOException.class, () -> gson.fromJson("{}", Record.class));
     assertThat(exception).hasMessageThat()
-            .isEqualTo("Abstract classes can't be instantiated! Register an InstanceCreator or a TypeAdapter for"
-                + " this type. Class name: java.lang.Record");
+        .isEqualTo("Abstract classes can't be instantiated! Adjust the R8 configuration or register an InstanceCreator"
+            + " or a TypeAdapter for this type. Class name: java.lang.Record"
+            + "\nSee https://github.com/google/gson/blob/main/Troubleshooting.md#r8-abstract-class");
   }
 }
diff --git a/gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java b/gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java
--- a/gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java
+++ b/gson/src/test/java/com/google/gson/internal/ConstructorConstructorTest.java
@@ -19,17 +19,14 @@
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.fail;
 
-import com.google.gson.InstanceCreator;
-import com.google.gson.ReflectionAccessFilter;
 import com.google.gson.reflect.TypeToken;
-import java.lang.reflect.Type;
 import java.util.Collections;
 import org.junit.Test;
 
 public class ConstructorConstructorTest {
   private ConstructorConstructor constructorConstructor = new ConstructorConstructor(
-      Collections.<Type, InstanceCreator<?>>emptyMap(), true,
-      Collections.<ReflectionAccessFilter>emptyList()
+      Collections.emptyMap(), true,
+      Collections.emptyList()
   );
 
   private abstract static class AbstractClass {
@@ -39,7 +36,7 @@ public AbstractClass() { }
   private interface Interface { }
 
   /**
-   * Verify that ConstructorConstructor does not try to invoke no-arg constructor
+   * Verify that ConstructorConstructor does not try to invoke no-args constructor
    * of abstract class.
    */
   @Test
@@ -49,9 +46,10 @@ public void testGet_AbstractClassNoArgConstructor() {
       constructor.construct();
       fail("Expected exception");
     } catch (RuntimeException exception) {
-      assertThat(exception).hasMessageThat().isEqualTo("Abstract classes can't be instantiated! "
-          + "Register an InstanceCreator or a TypeAdapter for this type. "
-          + "Class name: com.google.gson.internal.ConstructorConstructorTest$AbstractClass");
+      assertThat(exception).hasMessageThat().isEqualTo("Abstract classes can't be instantiated!"
+          + " Adjust the R8 configuration or register an InstanceCreator or a TypeAdapter for this type."
+          + " Class name: com.google.gson.internal.ConstructorConstructorTest$AbstractClass"
+          + "\nSee https://github.com/google/gson/blob/main/Troubleshooting.md#r8-abstract-class");
     }
   }
 
@@ -62,9 +60,9 @@ public void testGet_Interface() {
       constructor.construct();
       fail("Expected exception");
     } catch (RuntimeException exception) {
-      assertThat(exception).hasMessageThat().isEqualTo("Interfaces can't be instantiated! "
-          + "Register an InstanceCreator or a TypeAdapter for this type. "
-          + "Interface name: com.google.gson.internal.ConstructorConstructorTest$Interface");
+      assertThat(exception).hasMessageThat().isEqualTo("Interfaces can't be instantiated!"
+          + " Register an InstanceCreator or a TypeAdapter for this type."
+          + " Interface name: com.google.gson.internal.ConstructorConstructorTest$Interface");
     }
   }
 }
diff --git a/shrinker-test/src/test/java/com/google/gson/it/ShrinkingIT.java b/shrinker-test/src/test/java/com/google/gson/it/ShrinkingIT.java
--- a/shrinker-test/src/test/java/com/google/gson/it/ShrinkingIT.java
+++ b/shrinker-test/src/test/java/com/google/gson/it/ShrinkingIT.java
@@ -220,4 +220,24 @@ public void testDefaultConstructorNoJdkUnsafe() throws Exception {
       }
     });
   }
+
+  @Test
+  public void testNoDefaultConstructor() throws Exception {
+    runTest("com.example.DefaultConstructorMain", c -> {
+      Method m = c.getMethod("runTestNoDefaultConstructor");
+
+      if (jarToTest.equals(PROGUARD_RESULT_PATH)) {
+        Object result = m.invoke(null);
+        assertThat(result).isEqualTo("value");
+      } else {
+        // R8 performs more aggressive optimizations
+        Exception e = assertThrows(InvocationTargetException.class, () -> m.invoke(null));
+        assertThat(e).hasCauseThat().hasMessageThat().isEqualTo(
+            "Abstract classes can't be instantiated! Adjust the R8 configuration or register an InstanceCreator"
+            + " or a TypeAdapter for this type. Class name: com.example.DefaultConstructorMain$TestClassWithoutDefaultConstructor"
+            + "\nSee https://github.com/google/gson/blob/main/Troubleshooting.md#r8-abstract-class"
+        );
+      }
+    });
+  }
 }