IDEA-106884 Detect that string concatenation produces not-null result

java/java-impl/src/com/intellij/codeInspection/dataFlow/ControlFlowAnalyzer.java

@@ -182,6 +182,11 @@ class ControlFlowAnalyzer extends JavaElementVisitor {
           generateDefaultBinOp(lExpr, rExpr, type);
         }
       }
+      else if (op == JavaTokenType.PLUSEQ && type != null && type.equalsToText(JAVA_LANG_STRING)) {
+        lExpr.accept(this);
+        rExpr.accept(this);
+        addInstruction(new BinopInstruction(JavaTokenType.PLUS, null, lExpr.getProject()));
+      }
       else {
         generateDefaultBinOp(lExpr, rExpr, type);
       }

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

@@ -0,0 +1,15 @@
+import org.jetbrains.annotations.NotNull;
+
+class X {
+  @NotNull
+  String foo() {return "";}
+
+  void bar() {
+    String o = foo();
+    o += "";
+    if (<warning descr="Condition 'o != null' is always 'true'">o != null</warning>) {
+
+    }
+  }
+
+}

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

@@ -68,6 +68,7 @@ public class DataFlowInspectionTest extends LightCodeInsightFixtureTestCase {
   public void testIDEA84489() throws Throwable { doTest(); }
   public void testComparingToNotNullShouldNotAffectNullity() throws Throwable { doTest(); }
   public void testStringTernaryAlwaysTrue() throws Throwable { doTest(); }
+  public void testStringConcatAlwaysNotNull() throws Throwable { doTest(); }
 
   public void testNotNullPrimitive() throws Throwable { doTest(); }
   public void testBoxing128() throws Throwable { doTest(); }