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)

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.kt

@@ -0,0 +1,20 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+// FILE: J.java
+
+import org.jetbrains.annotations.*;
+
+public class J {
+    @Nullable
+    public static J staticN;
+}
+
+// FILE: k.kt
+
+fun test() {
+    val a = J.staticN <!USELESS_ELVIS_RIGHT_IS_NULL!>?: null<!>
+    foo(a)
+}
+
+fun foo(a: Any?) {
+}

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.txt

@@ -0,0 +1,14 @@
+package
+
+public fun foo(/*0*/ a: kotlin.Any?): kotlin.Unit
+public fun test(): kotlin.Unit
+
+public open class J {
+    public constructor J()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    @org.jetbrains.annotations.Nullable() public final var staticN: J?
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -13855,6 +13855,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisInCall.kt");
                     doTest(fileName);
                 }
+
+                @TestMetadata("uselessElvisRightIsNull.kt")
+                public void testUselessElvisRightIsNull() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.kt");
+                    doTest(fileName);
+                }
             }
 
             @TestMetadata("compiler/testData/diagnostics/tests/platformTypes/rawTypes")

idea/src/org/jetbrains/kotlin/idea/quickfix/QuickFixRegistrar.kt

@@ -98,6 +98,7 @@ class QuickFixRegistrar : QuickFixContributor {
         WRONG_GETTER_RETURN_TYPE.registerFactory(ChangeAccessorTypeFix)
 
         USELESS_ELVIS.registerFactory(RemoveUselessElvisFix)
+        USELESS_ELVIS_RIGHT_IS_NULL.registerFactory(RemoveUselessElvisFix)
 
         val removeRedundantModifierFactory = RemoveModifierFix.createRemoveModifierFactory(true)
         REDUNDANT_MODIFIER.registerFactory(removeRedundantModifierFactory)