KT-13521 Warning right part of "expression ?: null" is useless

#KT-13521 Fixed

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -686,6 +686,8 @@ public interface Errors {
 
     DiagnosticFactory1<KtBinaryExpression, KotlinType> USELESS_ELVIS = DiagnosticFactory1.create(WARNING, PositioningStrategies.USELESS_ELVIS);
     DiagnosticFactory0<PsiElement> USELESS_ELVIS_ON_LAMBDA_EXPRESSION = DiagnosticFactory0.create(WARNING);
+    DiagnosticFactory0<KtBinaryExpression> USELESS_ELVIS_RIGHT_IS_NULL =
+            DiagnosticFactory0.create(WARNING, PositioningStrategies.USELESS_ELVIS);
 
     // Compile-time values
 

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -418,6 +418,7 @@ public class DefaultErrorMessages {
         MAP.put(DYNAMIC_UPPER_BOUND, "Dynamic type can not be used as an upper bound");
         MAP.put(USELESS_ELVIS, "Elvis operator (?:) always returns the left operand of non-nullable type {0}", RENDER_TYPE);
         MAP.put(USELESS_ELVIS_ON_LAMBDA_EXPRESSION, "Left operand of elvis operator (?:) is a lambda expression");
+        MAP.put(USELESS_ELVIS_RIGHT_IS_NULL, "Right operand of elvis operator (?:) is useless if it is null");
         MAP.put(CONFLICTING_UPPER_BOUNDS, "Upper bounds of {0} have empty intersection", NAME);
 
         MAP.put(UNSUPPORTED_TYPEALIAS, "Type aliases are unsupported (min Kotlin language level: 1.1)");

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/BasicExpressionTypingVisitor.java

@@ -1198,6 +1198,9 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
         assert rightTypeInfo != null : "Right expression was not processed: " + expression;
         boolean loopBreakContinuePossible = leftTypeInfo.getJumpOutPossible() || rightTypeInfo.getJumpOutPossible();
         KotlinType rightType = rightTypeInfo.getType();
+        if (rightType != null && KtPsiUtil.isNullConstant(right)) {
+            context.trace.report(USELESS_ELVIS_RIGHT_IS_NULL.on(expression));
+        }
 
         // Only left argument DFA is taken into account here: we cannot be sure that right argument is joined
         // (we merge it with right DFA if right argument contains no jump outside)