enable contract checking when a notnull method is called (IDEA-CR-28918)

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

@@ -15,6 +15,7 @@
  */
 package com.intellij.codeInspection.dataFlow;
 
+import com.intellij.codeInsight.NullableNotNullManager;
 import com.intellij.codeInspection.dataFlow.instructions.CheckReturnValueInstruction;
 import com.intellij.codeInspection.dataFlow.instructions.Instruction;
 import com.intellij.codeInspection.dataFlow.instructions.MethodCallInstruction;
@@ -115,7 +116,7 @@ class ContractChecker extends DataFlowRunner {
 
   private static boolean weCannotInferAnythingAboutMethodReturnValue(MethodCallInstruction instruction) {
     PsiMethod target = instruction.getTargetMethod();
-    return instruction.getContracts().isEmpty() && target != null && !target.isConstructor();
+    return instruction.getContracts().isEmpty() && target != null && !target.isConstructor() && !NullableNotNullManager.isNotNull(target);
   }
 
   @NotNull

java/java-tests/testData/inspection/dataFlow/contractCheck/CallingNotNullMethod.java

@@ -0,0 +1,21 @@
+import org.jetbrains.annotations.*;
+
+class Test {
+  
+  @Contract("!null -> null")
+  static String test(String s) {
+    if(s != null) {
+      return <warning descr="Contract clause '!null -> null' is violated">getValue(s)</warning>;
+    }
+    return getDefaultValue();
+  }
+
+  @NotNull
+  static String getValue(String s) {
+    return s.trim();
+  }
+
+  static String getDefaultValue() {
+    return "foo";
+  }
+}

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

@@ -63,4 +63,5 @@ public class ContractCheckTest extends LightCodeInsightFixtureTestCase {
 
   public void testPassingVarargsToDelegate() { doTest(); }
   public void testUnknownIfCondition() { doTest(); }
+  public void testCallingNotNullMethod() { doTest(); }
 }