Disallow expressions of type void to be used in truthiness checks

src/compiler/checker.ts

@@ -20949,6 +20949,7 @@ namespace ts {
                     }
                     return numberType;
                 case SyntaxKind.ExclamationToken:
+                    checkTruthinessExpression(node.operand);
                     const facts = getTypeFacts(operandType) & (TypeFacts.Truthy | TypeFacts.Falsy);
                     return facts === TypeFacts.Truthy ? falseType :
                         facts === TypeFacts.Falsy ? trueType :
@@ -21316,7 +21317,14 @@ namespace ts {
             if (operator === SyntaxKind.EqualsToken && (left.kind === SyntaxKind.ObjectLiteralExpression || left.kind === SyntaxKind.ArrayLiteralExpression)) {
                 return checkDestructuringAssignment(left, checkExpression(right, checkMode), checkMode);
             }
-            let leftType = checkExpression(left, checkMode);
+            let leftType: Type;
+            if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.BarBarToken) {
+                leftType = checkTruthinessExpression(left, checkMode);
+            }
+            else {
+                leftType = checkExpression(left, checkMode);
+            }
+
             let rightType = checkExpression(right, checkMode);
             switch (operator) {
                 case SyntaxKind.AsteriskToken:
@@ -21649,7 +21657,7 @@ namespace ts {
         }
 
         function checkConditionalExpression(node: ConditionalExpression, checkMode?: CheckMode): Type {
-            checkExpression(node.condition);
+            checkTruthinessExpression(node.condition);
             const type1 = checkExpression(node.whenTrue, checkMode);
             const type2 = checkExpression(node.whenFalse, checkMode);
             return getUnionType([type1, type2], UnionReduction.Subtype);
@@ -24512,7 +24520,7 @@ namespace ts {
             // Grammar checking
             checkGrammarStatementInAmbientContext(node);
 
-            checkExpression(node.expression);
+            checkTruthinessExpression(node.expression);
             checkSourceElement(node.thenStatement);
 
             if (node.thenStatement.kind === SyntaxKind.EmptyStatement) {
@@ -24527,17 +24535,25 @@ namespace ts {
             checkGrammarStatementInAmbientContext(node);
 
             checkSourceElement(node.statement);
-            checkExpression(node.expression);
+            checkTruthinessExpression(node.expression);
         }
 
         function checkWhileStatement(node: WhileStatement) {
             // Grammar checking
             checkGrammarStatementInAmbientContext(node);
 
-            checkExpression(node.expression);
+            checkTruthinessExpression(node.expression);
             checkSourceElement(node.statement);
         }
 
+        function checkTruthinessExpression(node: Expression, checkMode?: CheckMode) {
+            const type = checkExpression(node, checkMode);
+            if (type.flags & TypeFlags.Void) {
+                error(node, Diagnostics.An_expression_of_type_void_cannot_be_tested_for_truthiness);
+            }
+            return type;
+        }
+
         function checkForStatement(node: ForStatement) {
             // Grammar checking
             if (!checkGrammarStatementInAmbientContext(node)) {
@@ -24555,7 +24571,7 @@ namespace ts {
                 }
             }
 
-            if (node.condition) checkExpression(node.condition);
+            if (node.condition) checkTruthinessExpression(node.condition);
             if (node.incrementor) checkExpression(node.incrementor);
             checkSourceElement(node.statement);
             if (node.locals) {

src/compiler/diagnosticMessages.json

@@ -987,6 +987,10 @@
         "category": "Error",
         "code": 1344
     },
+    "An expression of type 'void' cannot be tested for truthiness": {
+        "category": "Error",
+        "code": 1345
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

src/server/scriptInfo.ts

@@ -459,12 +459,15 @@ namespace ts.server {
             if (this.isDynamicOrHasMixedContent()) {
                 this.textStorage.reload("");
                 this.markContainingProjectsAsDirty();
+                return true;
             }
             else {
                 if (this.textStorage.reloadWithFileText(tempFileName)) {
                     this.markContainingProjectsAsDirty();
+                    return true;
                 }
             }
+            return false;
         }
 
         /*@internal*/

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -8114,7 +8114,7 @@ declare namespace ts.server {
         setOptions(formatSettings: FormatCodeSettings, preferences: UserPreferences | undefined): void;
         getLatestVersion(): string;
         saveTo(fileName: string): void;
-        reloadFromFile(tempFileName?: NormalizedPath): void;
+        reloadFromFile(tempFileName?: NormalizedPath): boolean;
         editContent(start: number, end: number, newText: string): void;
         markContainingProjectsAsDirty(): void;
         isOrphan(): boolean;

tests/baselines/reference/conditionalOperatorConditionIsObjectType.errors.txt

@@ -0,0 +1,82 @@
+tests/cases/conformance/expressions/conditonalOperator/conditionalOperatorConditionIsObjectType.ts(36,1): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/conditonalOperator/conditionalOperatorConditionIsObjectType.ts(39,1): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/conditonalOperator/conditionalOperatorConditionIsObjectType.ts(58,20): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/conditonalOperator/conditionalOperatorConditionIsObjectType.ts(61,23): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/conditonalOperator/conditionalOperatorConditionIsObjectType.ts(63,32): error TS1345: An expression of type 'void' cannot be tested for truthiness
+
+
+==== tests/cases/conformance/expressions/conditonalOperator/conditionalOperatorConditionIsObjectType.ts (5 errors) ====
+    //Cond ? Expr1 : Expr2,  Cond is of object type, Expr1 and Expr2 have the same type
+    var condObject: Object;
+
+    var exprAny1: any;
+    var exprBoolean1: boolean;
+    var exprNumber1: number;
+    var exprString1: string;
+    var exprIsObject1: Object;
+
+    var exprAny2: any;
+    var exprBoolean2: boolean;
+    var exprNumber2: number;
+    var exprString2: string;
+    var exprIsObject2: Object;
+
+    function foo() { };
+    class C { static doIt: () => void };
+
+    //Cond is an object type variable
+    condObject ? exprAny1 : exprAny2;
+    condObject ? exprBoolean1 : exprBoolean2;
+    condObject ? exprNumber1 : exprNumber2;
+    condObject ? exprString1 : exprString2;
+    condObject ? exprIsObject1 : exprIsObject2;
+    condObject ? exprString1 : exprBoolean1; // union
+
+    //Cond is an object type literal
+    ((a: string) => a.length) ? exprAny1 : exprAny2;
+    ((a: string) => a.length) ? exprBoolean1 : exprBoolean2;
+    ({}) ? exprNumber1 : exprNumber2;
+    ({ a: 1, b: "s" }) ? exprString1 : exprString2;
+    ({ a: 1, b: "s" }) ? exprIsObject1 : exprIsObject2;
+    ({ a: 1, b: "s" }) ? exprString1: exprBoolean1; // union
+
+    //Cond is an object type expression
+    foo() ? exprAny1 : exprAny2;
+    ~~~~~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    new Date() ? exprBoolean1 : exprBoolean2;
+    new C() ? exprNumber1 : exprNumber2;
+    C.doIt() ? exprString1 : exprString2;
+    ~~~~~~~~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    condObject.valueOf() ? exprIsObject1 : exprIsObject2;
+    new Date() ? exprString1 : exprBoolean1; // union
+
+    //Results shoud be same as Expr1 and Expr2
+    var resultIsAny1 = condObject ? exprAny1 : exprAny2;
+    var resultIsBoolean1 = condObject ? exprBoolean1 : exprBoolean2;
+    var resultIsNumber1 = condObject ? exprNumber1 : exprNumber2;
+    var resultIsString1 = condObject ? exprString1 : exprString2;
+    var resultIsObject1 = condObject ? exprIsObject1 : exprIsObject2;
+    var resultIsStringOrBoolean1 = condObject ? exprString1 : exprBoolean1; // union
+
+    var resultIsAny2 = ((a: string) => a.length) ? exprAny1 : exprAny2;
+    var resultIsBoolean2 = ((a: string) => a.length) ? exprBoolean1 : exprBoolean2;
+    var resultIsNumber2 = ({}) ? exprNumber1 : exprNumber2;
+    var resultIsString2 = ({ a: 1, b: "s" }) ? exprString1 : exprString2;
+    var resultIsObject2 = ({ a: 1, b: "s" }) ? exprIsObject1 : exprIsObject2;
+    var resultIsStringOrBoolean2 = ({ a: 1, b: "s" }) ? exprString1 : exprBoolean1; // union
+
+    var resultIsAny3 = foo() ? exprAny1 : exprAny2;
+                       ~~~~~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    var resultIsBoolean3 = new Date() ? exprBoolean1 : exprBoolean2;
+    var resultIsNumber3 = new C() ? exprNumber1 : exprNumber2;
+    var resultIsString3 = C.doIt() ? exprString1 : exprString2;
+                          ~~~~~~~~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    var resultIsObject3 = condObject.valueOf() ? exprIsObject1 : exprIsObject2;
+    var resultIsStringOrBoolean3 = C.doIt() ? exprString1 : exprBoolean1; // union
+                                   ~~~~~~~~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+

tests/baselines/reference/logicalAndOperatorStrictMode.errors.txt

@@ -0,0 +1,107 @@
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(46,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(47,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(48,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(49,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(50,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(51,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(52,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts(53,12): error TS1345: An expression of type 'void' cannot be tested for truthiness
+
+
+==== tests/cases/conformance/expressions/binaryOperators/logicalAndOperator/logicalAndOperatorStrictMode.ts (8 errors) ====
+    const a = [0];
+    const s = "";
+    const x = 0;
+    const b = false;
+    const v: void = undefined;
+    const u = undefined;
+    const n = null;
+    const z = s || x || u;
+
+    const a1 = a && a;
+    const a2 = a && s;
+    const a3 = a && x;
+    const a4 = a && b;
+    const a5 = a && v;
+    const a6 = a && u;
+    const a7 = a && n;
+    const a8 = a && z;
+
+    const s1 = s && a;
+    const s2 = s && s;
+    const s3 = s && x;
+    const s4 = s && b;
+    const s5 = s && v;
+    const s6 = s && u;
+    const s7 = s && n;
+    const s8 = s && z;
+
+    const x1 = x && a;
+    const x2 = x && s;
+    const x3 = x && x;
+    const x4 = x && b;
+    const x5 = x && v;
+    const x6 = x && u;
+    const x7 = x && n;
+    const x8 = x && z;
+
+    const b1 = b && a;
+    const b2 = b && s;
+    const b3 = b && x;
+    const b4 = b && b;
+    const b5 = b && v;
+    const b6 = b && u;
+    const b7 = b && n;
+    const b8 = b && z;
+
+    const v1 = v && a;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v2 = v && s;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v3 = v && x;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v4 = v && b;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v5 = v && v;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v6 = v && u;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v7 = v && n;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+    const v8 = v && z;
+               ~
+!!! error TS1345: An expression of type 'void' cannot be tested for truthiness
+
+    const u1 = u && a;
+    const u2 = u && s;
+    const u3 = u && x;
+    const u4 = u && b;
+    const u5 = u && v;
+    const u6 = u && u;
+    const u7 = u && n;
+    const u8 = u && z;
+
+    const n1 = n && a;
+    const n2 = n && s;
+    const n3 = n && x;
+    const n4 = n && b;
+    const n5 = n && v;
+    const n6 = n && u;
+    const n7 = n && n;
+    const n8 = n && z;
+
+    const z1 = z && a;
+    const z2 = z && s;
+    const z3 = z && x;
+    const z4 = z && b;
+    const z5 = z && v;
+    const z6 = z && u;
+    const z7 = z && n;
+    const z8 = z && z;