[java-psi] IDEA-377483 Wrong NPE message when iterating over Iterables.concat

Handle when `T extends @Nullable Type` is instantiated with unknown nullability

GitOrigin-RevId: 21d7cc928f4f5d9ac037fa42a632f936ed6c6516

java/java-psi-api/src/com/intellij/codeInsight/TypeNullability.java

@@ -78,6 +78,9 @@ public final class TypeNullability {
     if (this.nullability() == Nullability.NOT_NULL) {
       return this;
     }
+    if (this.nullability() == Nullability.NULLABLE && this.source() instanceof NullabilitySource.ExtendsBound) {
+      return nullability;
+    }
     if (nullability.nullability() == Nullability.NOT_NULL && this.source() instanceof NullabilitySource.ExtendsBound) {
       return nullability;
     }

java/java-tests/testData/inspection/dataFlow/fixture/InheritNullableInstantiateUnknown.java

@@ -0,0 +1,17 @@
+import org.jetbrains.annotations.NotNullByDefault;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.List;
+
+class MyTest {
+  @NotNullByDefault
+  static class Iterables {
+    public static native <T extends @Nullable Object> Iterable<T> concat(Iterable<? extends T> a, Iterable<? extends T> b);
+  }
+
+  void foo(List<String> l1, List<String> l2) {
+    for (String s : Iterables.concat(l1, l2)) {
+      System.out.println(s.length());
+    }
+  }
+}

java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection21Test.java

@@ -211,6 +211,10 @@ public class DataFlowInspection21Test extends DataFlowInspectionTestCase {
     setupTypeUseAnnotations("org.jspecify.annotations", myFixture);
     doTest();
   }
+  
+  public void testInheritNullableInstantiateUnknown() {
+    doTest();
+  }
 
   public void testPassthroughGenericParameter() {
     doTestWith((dfi, cvi) -> dfi.TREAT_UNKNOWN_MEMBERS_AS_NULLABLE = true);