KT-1680 Warn if non-null variable is compared to null

#KT-1680 fixed

compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java

@@ -380,6 +380,8 @@ public interface Errors {
         }
     }, TO_STRING, TO_STRING);
 
+    DiagnosticFactory2<JetBinaryExpression, JetBinaryExpression, Boolean> SENSELESS_COMPARISON = DiagnosticFactory2.create(WARNING, "Condition ''{0}'' is always ''{1}''", ELEMENT_TEXT, TO_STRING);
+
     DiagnosticFactory2<PsiElement, CallableMemberDescriptor, DeclarationDescriptor> OVERRIDING_FINAL_MEMBER = DiagnosticFactory2.create(ERROR, "''{0}'' in ''{1}'' is final and cannot be overridden", NAME, NAME);
     DiagnosticFactory3<JetModifierListOwner, Visibility, CallableMemberDescriptor, DeclarationDescriptor> CANNOT_WEAKEN_ACCESS_PRIVILEGE = DiagnosticFactory3.create(ERROR, "Cannot weaken access privilege ''{0}'' for ''{1}'' in ''{2}''", PositioningStrategies.POSITION_VISIBILITY_MODIFIER, TO_STRING, NAME, NAME);
     DiagnosticFactory3<JetModifierListOwner, Visibility, CallableMemberDescriptor, DeclarationDescriptor> CANNOT_CHANGE_ACCESS_PRIVILEGE = DiagnosticFactory3.create(ERROR, "Cannot change access privilege ''{0}'' for ''{1}'' in ''{2}''", PositioningStrategies.POSITION_VISIBILITY_MODIFIER, TO_STRING, NAME, NAME);

compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/BasicExpressionTypingVisitor.java

@@ -950,9 +950,16 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
                     context.trace.report(EQUALITY_NOT_APPLICABLE.on(expression, operationSign, leftType, rightType));
                 }
             }
+            if (isSenselessComparisonWithNull(leftType, right) || isSenselessComparisonWithNull(rightType, left)) {
+                context.trace.report(SENSELESS_COMPARISON.on(expression, expression, operationSign.getReferencedNameElementType() == JetTokens.EXCLEQ));
+            }
         }
     }
 
+    private boolean isSenselessComparisonWithNull(JetType firstType, JetExpression secondExpression) {
+        return !firstType.isNullable() && secondExpression instanceof JetConstantExpression && secondExpression.getNode().getElementType() == JetNodeTypes.NULL;
+    }
+
     protected JetType visitAssignmentOperation(JetBinaryExpression expression, ExpressionTypingContext context) {
         return assignmentIsNotAnExpressionError(expression, context);
     }

compiler/testData/diagnostics/tests/nullabilityAndAutoCasts/kt1680.jet

@@ -0,0 +1,14 @@
+//KT-1680 Warn if non-null variable is compared to null
+package kt1680
+
+fun foo() {
+    val x = 1
+    if (<!SENSELESS_COMPARISON!>x != null<!>) {} // <-- need a warning here!
+    if (<!SENSELESS_COMPARISON!>x == null<!>) {}
+    if (<!SENSELESS_COMPARISON!>null != x<!>) {}
+    if (<!SENSELESS_COMPARISON!>null == x<!>) {}
+
+    val y : Int? = 1
+    if (y != null) {}
+    if (y == null) {}
+}