All types are immutable for nullness. Field visibility shouldn't matter.

checker/src/main/java/org/checkerframework/checker/nullness/NullnessAnnotatedTypeFactory.java

@@ -126,7 +126,7 @@ public class NullnessAnnotatedTypeFactory
   /** Aliases for {@code @Nonnull}. */
   @SuppressWarnings({
     "signature:argument", // Class names intentionally obfuscated
-    "signature:assignment.type.incompatible" // Class names intentionally obfuscated
+    "signature:assignment" // Class names intentionally obfuscated
   })
   private static final List<@FullyQualifiedName String> NONNULL_ALIASES =
       Arrays.<@FullyQualifiedName String>asList(
@@ -221,7 +221,7 @@ public class NullnessAnnotatedTypeFactory
   /** Aliases for {@code @Nullable}. */
   @SuppressWarnings({
     "signature:argument", // Class names intentionally obfuscated
-    "signature:assignment.type.incompatible" // Class names intentionally obfuscated
+    "signature:assignment" // Class names intentionally obfuscated
   })
   private static final List<@FullyQualifiedName String> NULLABLE_ALIASES =
       Arrays.<@FullyQualifiedName String>asList(
@@ -356,7 +356,7 @@ public class NullnessAnnotatedTypeFactory
   /** Aliases for {@code @PolyNull}. */
   @SuppressWarnings({
     "signature:argument", // Class names intentionally obfuscated
-    "signature:assignment.type.incompatible" // Class names intentionally obfuscated
+    "signature:assignment" // Class names intentionally obfuscated
   })
   private static final List<@FullyQualifiedName String> POLYNULL_ALIASES =
       Arrays.<@FullyQualifiedName String>asList(

checker/src/main/java/org/checkerframework/checker/units/UnitsAnnotatedTypeFactory.java

@@ -611,8 +611,11 @@ protected AnnotationMirror leastUpperBoundWithElements(
       throw new TypeSystemError("Unexpected QualifierKinds: %s %s", qualifierKind1, qualifierKind2);
     }
 
-    @SuppressWarnings("nullness:return")
     @Override
+    @SuppressWarnings(
+        "nullness:return" // This class UnitsQualifierHierarchy is annotated for nullness,
+    // but the outer class UnitsAnnotatedTypeFactory is not, so the type of fields is @Nullable.
+    )
     protected AnnotationMirror greatestLowerBoundWithElements(
         AnnotationMirror a1,
         QualifierKind qualifierKind1,

checker/tests/ainfer-testchecker/non-annotated/InterfaceTest.java

@@ -13,7 +13,6 @@ public interface InterfaceTest {
   default void requireAinferSibling1(@AinferSibling1 String x) {}
 
   default void testX() {
-    // :: warning: (argument)
     requireAinferSibling1(toaster);
   }
 }

checker/tests/nullness/Issue4923.java

@@ -0,0 +1,19 @@
+class Issue4923 {
+  interface Go {
+    void go();
+  }
+
+  final Go go =
+      new Go() {
+        @Override
+        public void go() {
+          synchronized (x) {
+          }
+        }
+      };
+  final Object x = new Object();
+
+  // Make sure that initializer type is compatible with declared type
+  // :: error: (assignment)
+  final Object y = null;
+}

framework/src/main/java/org/checkerframework/framework/flow/CFAbstractTransfer.java

@@ -20,7 +20,6 @@
 import javax.lang.model.element.Element;
 import javax.lang.model.element.ElementKind;
 import javax.lang.model.element.ExecutableElement;
-import javax.lang.model.element.Modifier;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.element.VariableElement;
 import javax.lang.model.type.TypeMirror;
@@ -471,7 +470,8 @@ private boolean isExpressionOrStatementPure(
   /**
    * Add field values to the initial store before {@code methodTree}.
    *
-   * <p>The initializer value is inserted into {@code store} if the field is private and final.
+   * <p>The initializer value is inserted into {@code store} if the field is final and the field
+   * type is immutable, as defined by {@link AnnotatedTypeFactory#isImmutable(TypeMirror)}.
    *
    * <p>The declared value is inserted into {@code store} if:
    *
@@ -487,14 +487,16 @@ private boolean isExpressionOrStatementPure(
    * @param classTree the class that contains {@code methodTree}
    * @param methodTree the method or constructor tree
    */
+  // TODO: should field visibility matter? An access from outside the class might observe
+  // the declared type instead of a refined type. Issue a warning to alert users?
   private void addInitialFieldValues(S store, ClassTree classTree, MethodTree methodTree) {
     boolean isConstructor = TreeUtils.isConstructor(methodTree);
     TypeElement classEle = TreeUtils.elementFromDeclaration(classTree);
     for (FieldInitialValue<V> fieldInitialValue : analysis.getFieldInitialValues()) {
       VariableElement varEle = fieldInitialValue.fieldDecl.getField();
       // Insert the value from the initializer of private final fields.
       if (fieldInitialValue.initializer != null
-          && varEle.getModifiers().contains(Modifier.PRIVATE)
+          // && varEle.getModifiers().contains(Modifier.PRIVATE)
           && ElementUtils.isFinal(varEle)
           && analysis.atypeFactory.isImmutable(ElementUtils.getType(varEle))) {
         store.insertValue(fieldInitialValue.fieldDecl, fieldInitialValue.initializer);