qmljs: add check for comparisons not depending on values

currently we mainly try to warn about primitive == null/undefined or
primitive === non primitive.
There are other that we could warn about null==null null==undefined,
but I feel that they might be triggered too much by clean code.

Change-Id: Id43d838d60a4e13f361be34e4bb38211777a081e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/libs/qmljs/qmljscheck.cpp

@@ -1275,6 +1275,30 @@ static bool shouldAvoidNonStrictEqualityCheck(const Value *lhs, const Value *rhs
     return false;
 }
 
+static bool equalIsAlwaysFalse(const Value *lhs, const Value *rhs)
+{
+    if ((lhs->asNullValue() || lhs->asUndefinedValue())
+        && (rhs->asNumberValue() || rhs->asBooleanValue() || rhs->asStringValue()))
+        return true;
+    return false;
+}
+
+static bool strictCompareConstant(const Value *lhs, const Value *rhs)
+{
+    if (lhs->asUnknownValue() || rhs->asUnknownValue())
+        return false;
+    if (lhs->asBooleanValue() && !rhs->asBooleanValue())
+        return true;
+    if (lhs->asNumberValue() && !rhs->asNumberValue())
+        return true;
+    if (lhs->asStringValue() && !rhs->asStringValue())
+        return true;
+    if (lhs->asObjectValue() && (!rhs->asObjectValue() || !rhs->asNullValue() || !rhs->asUndefinedValue()))
+        return true;
+    return false;
+}
+
+
 bool Check::visit(BinaryExpression *ast)
 {
     const QString source = _doc->source();
@@ -1300,6 +1324,18 @@ bool Check::visit(BinaryExpression *ast)
                 || shouldAvoidNonStrictEqualityCheck(rhsValue, lhsValue)) {
             addMessage(MaybeWarnEqualityTypeCoercion, ast->operatorToken);
         }
+        if (equalIsAlwaysFalse(lhsValue, rhsValue)
+            || equalIsAlwaysFalse(rhsValue, lhsValue))
+            addMessage(WarnLogicalValueDoesNotDependOnValues, ast->operatorToken);
+    }
+    if (ast->op == QSOperator::StrictEqual || ast->op == QSOperator::StrictNotEqual) {
+        Evaluate eval(&_scopeChain);
+        const Value *lhsValue = eval(ast->left);
+        const Value *rhsValue = eval(ast->right);
+        if (strictCompareConstant(lhsValue, rhsValue)
+                || strictCompareConstant(rhsValue, lhsValue)) {
+            addMessage(WarnLogicalValueDoesNotDependOnValues, ast->operatorToken);
+        }
     }
 
     // check odd + ++ combinations

src/libs/qmljs/qmljsstaticanalysismessage.cpp

@@ -247,6 +247,8 @@ StaticAnalysisMessages::StaticAnalysisMessages()
            tr("Hit maximum recursion limit when visiting AST."));
     newMsg(ErrTypeIsInstantiatedRecursively, Error,
             tr("Type cannot be instantiated recursively (%1)."), 1);
+    newMsg(WarnLogicalValueDoesNotDependOnValues, Warning,
+           tr("Logical value does not depend on actual values"));
 }
 
 } // anonymous namespace

src/libs/qmljs/qmljsstaticanalysismessage.h

@@ -131,6 +131,7 @@ enum Type
     ErrShorterStringValueExpected = 322,
     ErrInvalidArrayValueLength = 323,
     ErrHitMaximumRecursion = 324,
+    WarnLogicalValueDoesNotDependOnValues = 325,
     WarnDuplicateImport = 400
 };