DfaPsiUtil#getFunctionalParameterNullability: use known type nullability for SAM parameters

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/DfaPsiUtil.java

@@ -30,6 +30,7 @@ import com.intellij.psi.tree.IElementType;
 import com.intellij.psi.util.*;
 import com.intellij.util.ArrayUtil;
 import com.intellij.util.NullableFunction;
+import com.intellij.util.ObjectUtils;
 import com.intellij.util.containers.ContainerUtil;
 import com.intellij.util.containers.JBIterable;
 import com.intellij.util.containers.MultiMap;
@@ -129,9 +130,16 @@ public class DfaPsiUtil {
     if(nullness != Nullness.UNKNOWN) {
       return nullness;
     }
-    PsiMethod sam = LambdaUtil.getFunctionalInterfaceMethod(function.getFunctionalInterfaceType());
+    PsiClassType type = ObjectUtils.tryCast(function.getFunctionalInterfaceType(), PsiClassType.class);
+    PsiMethod sam = LambdaUtil.getFunctionalInterfaceMethod(type);
     if (sam != null && index < sam.getParameterList().getParametersCount()) {
-      return getElementNullability(null, sam.getParameterList().getParameters()[index]);
+      PsiParameter parameter = sam.getParameterList().getParameters()[index];
+      nullness = getElementNullability(null, parameter);
+      if(nullness != Nullness.UNKNOWN) {
+        return nullness;
+      }
+      PsiType parameterType = type.resolveGenerics().getSubstitutor().substitute(parameter.getType());
+      return getTypeNullability(parameterType);
     }
     return Nullness.UNKNOWN;
   }

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

@@ -28,6 +28,10 @@ public class MethodReferenceConstantValue {
     }
   }
 
+  public void test(List<@foo.NotNull String> list) {
+    list.removeIf(<warning descr="Method reference result is always 'false'">Objects::isNull</warning>);
+  }
+
   interface X {
     boolean action(@Nullable MethodReferenceConstantValue a, @NotNull String b);
   }

java/java-tests/testSrc/com/intellij/codeInspection/DataFlowInspection8Test.java

@@ -60,7 +60,7 @@ public class DataFlowInspection8Test extends DataFlowInspectionTestCase {
   public void testNullableVoidLambda() { doTest(); }
   public void testNullableForeachVariable() { doTestWithCustomAnnotations(); }
   public void testGenericParameterNullity() { doTestWithCustomAnnotations(); }
-  public void testMethodReferenceConstantValue() { doTest(); }
+  public void testMethodReferenceConstantValue() { doTestWithCustomAnnotations(); }
 
   public void testOptionalOfNullable() { doTest(); }
   public void testOptionalOrElse() { doTest(); }