IDEA-163213 Method contract is not inferred when it returns the result of string concatenation

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

@@ -117,17 +117,7 @@ class ContractInferenceInterpreter {
 
     IElementType type = expr.getTokenType();
     if (type == POLYADIC_EXPRESSION || type == BINARY_EXPRESSION) {
-      List<LighterASTNode> operands = getExpressionChildren(myTree, expr);
-      if (operands.size() == 2) {
-        boolean equality = firstChildOfType(myTree, expr, JavaTokenType.EQEQ) != null;
-        if (equality || firstChildOfType(myTree, expr, JavaTokenType.NE) != null) {
-          return asPreContracts(visitEqualityComparison(states, operands.get(0), operands.get(1), equality));
-        }
-      }
-      boolean logicalAnd = firstChildOfType(myTree, expr, JavaTokenType.ANDAND) != null;
-      if (logicalAnd || firstChildOfType(myTree, expr, JavaTokenType.OROR) != null) {
-        return asPreContracts(visitLogicalOperation(operands, logicalAnd, states));
-      }
+      return visitPolyadic(states, expr);
     }
 
     if (type == CONDITIONAL_EXPRESSION) {
@@ -190,6 +180,26 @@ class ContractInferenceInterpreter {
     return Collections.emptyList();
   }
 
+  @NotNull
+  private List<PreContract> visitPolyadic(List<ValueConstraint[]> states, @NotNull LighterASTNode expr) {
+    if (firstChildOfType(myTree, expr, JavaTokenType.PLUS) != null) {
+      return asPreContracts(ContainerUtil.map(states, s -> new MethodContract(s, NOT_NULL_VALUE)));
+    }
+
+    List<LighterASTNode> operands = getExpressionChildren(myTree, expr);
+    if (operands.size() == 2) {
+      boolean equality = firstChildOfType(myTree, expr, JavaTokenType.EQEQ) != null;
+      if (equality || firstChildOfType(myTree, expr, JavaTokenType.NE) != null) {
+        return asPreContracts(visitEqualityComparison(states, operands.get(0), operands.get(1), equality));
+      }
+    }
+    boolean logicalAnd = firstChildOfType(myTree, expr, JavaTokenType.ANDAND) != null;
+    if (logicalAnd || firstChildOfType(myTree, expr, JavaTokenType.OROR) != null) {
+      return asPreContracts(visitLogicalOperation(operands, logicalAnd, states));
+    }
+    return Collections.emptyList();
+  }
+
   @NotNull
   private static List<PreContract> asPreContracts(List<MethodContract> contracts) {
     return ContainerUtil.map(contracts, KnownContract::new);

java/java-tests/testSrc/com/intellij/codeInspection/ContractInferenceFromSourceTest.groovy

@@ -438,6 +438,24 @@ class ContractInferenceFromSourceTest extends LightCodeInsightFixtureTestCase {
     assert c == ['null -> null']
   }
 
+  void "test string concatenation"() {
+    def c = inferContracts("""
+  public static String test(String s1, String s2) {
+    return s1 != null ? s1.trim()+s2.trim() : unknown();
+  }
+    """)
+    assert c == ['!null, _ -> !null']
+  }
+
+  void "test int addition"() {
+    def c = inferContracts("""
+  public static int test(int a, int b) {
+    return a + b;
+  }
+    """)
+    assert c == []
+  }
+
   void "test compare with string literal"() {
     def c = inferContracts("""
   String s(String s) {

java/java-tests/testSrc/com/intellij/codeInspection/NullityInferenceFromSourceTestCase.groovy

@@ -73,6 +73,10 @@ String bar() { return "z"; }
     assert inferNullity(parse('String bar() { return equals(2) ? "a" : equals(3) ? null : "a"; }; ')) == NULLABLE
   }
 
+  void "test string concatenation"() {
+    assert inferNullity(parse('String bar(String s1, String s2) { return s1 + s2; }; ')) == NOT_NULL
+  }
+
   void "test delegation to nullable means nothing"() {
     assert inferNullity(parse('String foo() { return bar("2"); }; String bar(String s) { if (s != "2") return null; return "a"; }; ')) == UNKNOWN
   }