Add shrinker-test examples with never-referenced constructors

R8 can do some optimizations that assume values of types whose
constructors are never referenced are `null`. (A bit confused why
the rules here are sufficient to avoid that optimization in the
case of a class whose sole constructor has arguments and is
unreferenced, since the keep rule only refers to `<init>()` instead
of `<init>(...)`. Does R8 have special behavior for `<init>()` where
attempting to keep that makes it assume a class is constructable even
if it doesn't have a no-args constructor?)

Also rename some test classes/names to refer to `NoArgs` instead of
`Default` constructors. The examples here care about whether a
no-argument constructor is present. While a no-argument
constructor (explicitly defined or not) is used by default in some
circumstances, the Java documentation consistently uses "default
constructor" to refer only to constructors whose implementation is
implicitly provided by the complier (all default constructors are
no-argument constructors, but not vice versa).

shrinker-test/common.pro

@@ -18,7 +18,7 @@
 -keep class com.example.NoSerializedNameMain {
   public static java.lang.String runTest();
   public static java.lang.String runTestNoJdkUnsafe();
-  public static java.lang.String runTestNoDefaultConstructor();
+  public static java.lang.String runTestConstructorHasArgs();
 }
 
 

shrinker-test/proguard.pro

@@ -12,6 +12,6 @@
 -keepclassmembernames class com.example.NoSerializedNameMain$TestClassNotAbstract {
   <fields>;
 }
--keepclassmembernames class com.example.NoSerializedNameMain$TestClassWithoutDefaultConstructor {
+-keepclassmembernames class com.example.NoSerializedNameMain$TestClassConstructorHasArgs {
   <fields>;
 }

shrinker-test/r8.pro

@@ -11,7 +11,7 @@
 
 # Don't obfuscate class name, to check it in exception message
 -keep,allowshrinking,allowoptimization class com.example.NoSerializedNameMain$TestClass
--keep,allowshrinking,allowoptimization class com.example.NoSerializedNameMain$TestClassWithoutDefaultConstructor
+-keep,allowshrinking,allowoptimization class com.example.NoSerializedNameMain$TestClassConstructorHasArgs
 
 # 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.NoSerializedNameMain$TestClassNotAbstract {

shrinker-test/src/main/java/com/example/ClassWithoutDefaultConstructor.java → shrinker-test/src/main/java/com/example/ClassWithHasArgsConstructor.java

@@ -3,15 +3,15 @@
 import com.google.gson.annotations.SerializedName;
 
 /**
- * Class without no-args default constructor, but with field annotated with
+ * Class without no-args constructor, but with field annotated with
  * {@link SerializedName}.
  */
-public class ClassWithoutDefaultConstructor {
+public class ClassWithHasArgsConstructor {
   @SerializedName("myField")
   public int i;
 
   // Specify explicit constructor with args to remove implicit no-args default constructor
-  public ClassWithoutDefaultConstructor(int i) {
+  public ClassWithHasArgsConstructor(int i) {
     this.i = i;
   }
 }

shrinker-test/src/main/java/com/example/ClassWithDefaultConstructor.java → shrinker-test/src/main/java/com/example/ClassWithNoArgsConstructor.java

@@ -6,11 +6,11 @@
  * Class with no-args default constructor and with field annotated with
  * {@link SerializedName}.
  */
-public class ClassWithDefaultConstructor {
+public class ClassWithNoArgsConstructor {
   @SerializedName("myField")
   public int i;
 
-  public ClassWithDefaultConstructor() {
+  public ClassWithNoArgsConstructor() {
     i = -3;
   }
 }

shrinker-test/src/main/java/com/example/ClassWithUnreferencedHasArgsConstructor.java

@@ -0,0 +1,19 @@
+package com.example;
+
+import com.google.gson.annotations.SerializedName;
+
+/**
+ * Class without no-args constructor, but with field annotated with
+ * {@link SerializedName}. The constructor should not actually be used in the
+ * code, but this shouldn't lead to R8 concluding that values of the type are
+ * not constructible and therefore must be null.
+ */
+public class ClassWithUnreferencedHasArgsConstructor {
+  @SerializedName("myField")
+  public int i;
+
+  // Specify explicit constructor with args to remove implicit no-args default constructor
+  public ClassWithUnreferencedHasArgsConstructor(int i) {
+    this.i = i;
+  }
+}

shrinker-test/src/main/java/com/example/ClassWithUnreferencedNoArgsConstructor.java

@@ -0,0 +1,18 @@
+package com.example;
+
+import com.google.gson.annotations.SerializedName;
+
+/**
+ * Class with no-args constructor and with field annotated with
+ * {@link SerializedName}. The constructor should not actually be used in the
+ * code, but this shouldn't lead to R8 concluding that values of the type are
+ * not constructible and therefore must be null.
+ */
+public class ClassWithUnreferencedNoArgsConstructor {
+  @SerializedName("myField")
+  public int i;
+
+  public ClassWithUnreferencedNoArgsConstructor() {
+    i = -3;
+  }
+}

shrinker-test/src/main/java/com/example/Main.java

@@ -32,7 +32,11 @@ public static void runTests(BiConsumer<String, String> outputConsumer) {
     testNamedFields(outputConsumer);
     testSerializedName(outputConsumer);
 
-    testNoDefaultConstructor(outputConsumer);
+    testConstructorNoArgs(outputConsumer);
+    testConstructorHasArgs(outputConsumer);
+    testUnreferencedConstructorNoArgs(outputConsumer);
+    testUnreferencedConstructorHasArgs(outputConsumer);
+
     testNoJdkUnsafe(outputConsumer);
 
     testEnum(outputConsumer);
@@ -90,24 +94,71 @@ private static void testSerializedName(BiConsumer<String, String> outputConsumer
     });
   }
 
-  private static void testNoDefaultConstructor(BiConsumer<String, String> outputConsumer) {
+  private static void testConstructorNoArgs(BiConsumer<String, String> outputConsumer) {
+    Gson gson = new GsonBuilder().setPrettyPrinting().create();
+    TestExecutor.run(outputConsumer, "Write: No args constructor", () -> toJson(
+        gson, new ClassWithNoArgsConstructor()));
+    TestExecutor.run(outputConsumer, "Read: No args constructor; initial constructor value", () -> {
+      ClassWithNoArgsConstructor deserialized = fromJson(gson, "{}", ClassWithNoArgsConstructor.class);
+      return Integer.toString(deserialized.i);
+    });
+    TestExecutor.run(outputConsumer, "Read: No args constructor; custom value", () -> {
+      ClassWithNoArgsConstructor deserialized = fromJson(gson, "{\"myField\": 3}", ClassWithNoArgsConstructor.class);
+      return Integer.toString(deserialized.i);
+    });
+  }
+
+  private static void testConstructorHasArgs(BiConsumer<String, String> outputConsumer) {
     Gson gson = new GsonBuilder().setPrettyPrinting().create();
-    TestExecutor.run(outputConsumer, "Write: No default constructor", () -> toJson(gson, new ClassWithoutDefaultConstructor(2)));
+    TestExecutor.run(outputConsumer, "Write: Constructor with args", () -> toJson(
+        gson, new ClassWithHasArgsConstructor(2)));
+    // This most likely relies on JDK Unsafe (unless the shrinker rewrites the constructor in some way)
+    TestExecutor.run(outputConsumer, "Read: Constructor with args", () -> {
+      ClassWithHasArgsConstructor deserialized = fromJson(
+          gson, "{\"myField\": 3}", ClassWithHasArgsConstructor.class);
+      return Integer.toString(deserialized.i);
+    });
+  }
+
+  private static void testUnreferencedConstructorNoArgs(BiConsumer<String, String> outputConsumer) {
+    Gson gson = new GsonBuilder().setPrettyPrinting().create();
+    // No write because we're not referencing this class's constructor.
+
+    // This runs the no-args constructor.
+    TestExecutor.run(outputConsumer, "Read: Unreferenced no args constructor; initial constructor value", () -> {
+      ClassWithUnreferencedNoArgsConstructor deserialized = fromJson(
+          gson, "{}", ClassWithUnreferencedNoArgsConstructor.class);
+      return Integer.toString(deserialized.i);
+    });
+    TestExecutor.run(outputConsumer, "Read: Unreferenced no args constructor; custom value", () -> {
+      ClassWithUnreferencedNoArgsConstructor deserialized = fromJson(
+          gson, "{\"myField\": 3}", ClassWithUnreferencedNoArgsConstructor.class);
+      return Integer.toString(deserialized.i);
+    });
+  }
+
+  private static void testUnreferencedConstructorHasArgs(BiConsumer<String, String> outputConsumer) {
+    Gson gson = new GsonBuilder().setPrettyPrinting().create();
+    // No write because we're not referencing this class's constructor.
+
     // This most likely relies on JDK Unsafe (unless the shrinker rewrites the constructor in some way)
-    TestExecutor.run(outputConsumer, "Read: No default constructor", () -> {
-      ClassWithoutDefaultConstructor deserialized = fromJson(gson, "{\"myField\": 3}", ClassWithoutDefaultConstructor.class);
+    TestExecutor.run(outputConsumer, "Read: Unreferenced constructor with args", () -> {
+      ClassWithUnreferencedHasArgsConstructor deserialized = fromJson(
+          gson, "{\"myField\": 3}", ClassWithUnreferencedHasArgsConstructor.class);
       return Integer.toString(deserialized.i);
     });
   }
 
   private static void testNoJdkUnsafe(BiConsumer<String, String> outputConsumer) {
     Gson gson = new GsonBuilder().disableJdkUnsafe().create();
     TestExecutor.run(outputConsumer, "Read: No JDK Unsafe; initial constructor value", () -> {
-      ClassWithDefaultConstructor deserialized = fromJson(gson, "{}", ClassWithDefaultConstructor.class);
+      ClassWithNoArgsConstructor deserialized = fromJson(
+          gson, "{}", ClassWithNoArgsConstructor.class);
       return Integer.toString(deserialized.i);
     });
     TestExecutor.run(outputConsumer, "Read: No JDK Unsafe; custom value", () -> {
-      ClassWithDefaultConstructor deserialized = fromJson(gson, "{\"myField\": 3}", ClassWithDefaultConstructor.class);
+      ClassWithNoArgsConstructor deserialized = fromJson(
+          gson, "{\"myField\": 3}", ClassWithNoArgsConstructor.class);
       return Integer.toString(deserialized.i);
     });
   }

shrinker-test/src/main/java/com/example/NoSerializedNameMain.java

@@ -19,25 +19,25 @@ static class TestClassNotAbstract {
     public String s;
   }
 
-  static class TestClassWithoutDefaultConstructor {
+  static class TestClassConstructorHasArgs {
     public String s;
 
     // Specify explicit constructor with args to remove implicit no-args default constructor
-    public TestClassWithoutDefaultConstructor(String s) {
+    public TestClassConstructorHasArgs(String s) {
       this.s = s;
     }
   }
 
   /**
-   * Main entrypoint, called by {@code ShrinkingIT.testNoSerializedName_DefaultConstructor()}.
+   * Main entrypoint, called by {@code ShrinkingIT.testNoSerializedName_ConstructorHasArgs()}.
    */
   public static String runTest() {
     TestClass deserialized = new Gson().fromJson("{\"s\":\"value\"}", same(TestClass.class));
     return deserialized.s;
   }
 
   /**
-   * Main entrypoint, called by {@code ShrinkingIT.testNoSerializedName_DefaultConstructorNoJdkUnsafe()}.
+   * Main entrypoint, called by {@code ShrinkingIT.testNoSerializedName_NoArgsConstructorNoJdkUnsafe()}.
    */
   public static String runTestNoJdkUnsafe() {
     Gson gson = new GsonBuilder().disableJdkUnsafe().create();
@@ -46,10 +46,10 @@ public static String runTestNoJdkUnsafe() {
   }
 
   /**
-   * Main entrypoint, called by {@code ShrinkingIT.testNoSerializedName_NoDefaultConstructor()}.
+   * Main entrypoint, called by {@code ShrinkingIT.testNoSerializedName_ConstructorHasArgs()}.
    */
-  public static String runTestNoDefaultConstructor() {
-    TestClassWithoutDefaultConstructor deserialized = new Gson().fromJson("{\"s\":\"value\"}", same(TestClassWithoutDefaultConstructor.class));
+  public static String runTestConstructorHasArgs() {
+    TestClassConstructorHasArgs deserialized = new Gson().fromJson("{\"s\":\"value\"}", same(TestClassConstructorHasArgs.class));
     return deserialized.s;
   }
 }

shrinker-test/src/test/java/com/google/gson/it/ShrinkingIT.java

@@ -129,12 +129,32 @@ public void test() throws Exception {
         "Read: SerializedName",
         "3",
         "===",
-        "Write: No default constructor",
+        "Write: No args constructor",
+        "{",
+        "  \"myField\": -3",
+        "}",
+        "===",
+        "Read: No args constructor; initial constructor value",
+        "-3",
+        "===",
+        "Read: No args constructor; custom value",
+        "3",
+        "===",
+        "Write: Constructor with args",
         "{",
         "  \"myField\": 2",
         "}",
         "===",
-        "Read: No default constructor",
+        "Read: Constructor with args",
+        "3",
+        "===",
+        "Read: Unreferenced no args constructor; initial constructor value",
+        "-3",
+        "===",
+        "Read: Unreferenced no args constructor; custom value",
+        "3",
+        "===",
+        "Read: Unreferenced constructor with args",
         "3",
         "===",
         "Read: No JDK Unsafe; initial constructor value",
@@ -191,7 +211,7 @@ public void test() throws Exception {
   }
 
   @Test
-  public void testNoSerializedName_DefaultConstructor() throws Exception {
+  public void testNoSerializedName_NoArgsConstructor() throws Exception {
     runTest("com.example.NoSerializedNameMain", c -> {
       Method m = c.getMethod("runTest");
 
@@ -211,7 +231,7 @@ public void testNoSerializedName_DefaultConstructor() throws Exception {
   }
 
   @Test
-  public void testNoSerializedName_DefaultConstructorNoJdkUnsafe() throws Exception {
+  public void testNoSerializedName_NoArgsConstructorNoJdkUnsafe() throws Exception {
     runTest("com.example.NoSerializedNameMain", c -> {
       Method m = c.getMethod("runTestNoJdkUnsafe");
 
@@ -232,9 +252,9 @@ public void testNoSerializedName_DefaultConstructorNoJdkUnsafe() throws Exceptio
   }
 
   @Test
-  public void testNoSerializedName_NoDefaultConstructor() throws Exception {
+  public void testNoSerializedName_ConstructorHasArgs() throws Exception {
     runTest("com.example.NoSerializedNameMain", c -> {
-      Method m = c.getMethod("runTestNoDefaultConstructor");
+      Method m = c.getMethod("runTestConstructorHasArgs");
 
       if (jarToTest.equals(PROGUARD_RESULT_PATH)) {
         Object result = m.invoke(null);
@@ -244,7 +264,7 @@ public void testNoSerializedName_NoDefaultConstructor() throws Exception {
         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.NoSerializedNameMain$TestClassWithoutDefaultConstructor"
+            + " or a TypeAdapter for this type. Class name: com.example.NoSerializedNameMain$TestClassConstructorHasArgs"
             + "\nSee https://github.com/google/gson/blob/main/Troubleshooting.md#r8-abstract-class"
         );
       }