Disallow truthiness checks on certain literal kinds

src/compiler/checker.ts

@@ -43948,6 +43948,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
         function helper(condExpr: Expression, body: Expression | Statement | undefined) {
             const location = isLogicalOrCoalescingBinaryExpression(condExpr) ? skipParentheses(condExpr.right) : condExpr;
+
             if (isModuleExportsAccessExpression(location)) {
                 return;
             }
@@ -44077,9 +44078,32 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     }
 
     function checkTruthinessOfType(type: Type, node: Node) {
+        node = skipParentheses(node);
         if (type.flags & TypeFlags.Void) {
             error(node, Diagnostics.An_expression_of_type_void_cannot_be_tested_for_truthiness);
         }
+
+        switch (node.kind) {
+            case SyntaxKind.NumericLiteral:
+                // Allow e.g. while(0) or while(1)
+                if ((node as NumericLiteral).text === "0" || (node as NumericLiteral).text === "1") break;
+            // falls through
+            case SyntaxKind.ArrayLiteralExpression:
+            case SyntaxKind.ArrowFunction:
+            case SyntaxKind.BigIntLiteral:
+            case SyntaxKind.ClassExpression:
+            case SyntaxKind.FunctionExpression:
+            case SyntaxKind.JsxElement:
+            case SyntaxKind.JsxSelfClosingElement:
+            case SyntaxKind.NoSubstitutionTemplateLiteral:
+            case SyntaxKind.ObjectLiteralExpression:
+            case SyntaxKind.RegularExpressionLiteral:
+            case SyntaxKind.StringLiteral:
+            case SyntaxKind.TemplateExpression:
+            case SyntaxKind.VoidExpression:
+                error(node, Diagnostics.Truthiness_test_of_this_kind_of_expression_appears_unintentional);
+        }
+
         return type;
     }
 

src/compiler/diagnosticMessages.json

@@ -3907,7 +3907,10 @@
         "category": "Error",
         "code": 2868
     },
-
+    "Truthiness test of this kind of expression appears unintentional": {
+        "category": "Error",
+        "code": 2869
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000

tests/baselines/reference/checkJsdocReturnTag1.errors.txt

@@ -0,0 +1,28 @@
+returns.js(20,12): error TS2869: Truthiness test of this kind of expression appears unintentional
+
+
+==== returns.js (1 errors) ====
+    // @ts-check
+    /**
+     * @returns {string} This comment is not currently exposed
+     */
+    function f() {
+        return "hello";
+    }
+
+    /**
+     * @returns {string=} This comment is not currently exposed
+     */
+    function f1() {
+        return "hello world";
+    }
+
+    /**
+     * @returns {string|number} This comment is not currently exposed
+     */
+    function f2() {
+        return 5 || "hello";
+               ~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    }
+

tests/baselines/reference/checkJsdocReturnTag2.errors.txt

@@ -1,9 +1,10 @@
 returns.js(6,5): error TS2322: Type 'number' is not assignable to type 'string'.
 returns.js(13,5): error TS2322: Type 'number | boolean' is not assignable to type 'string | number'.
   Type 'boolean' is not assignable to type 'string | number'.
+returns.js(13,12): error TS2869: Truthiness test of this kind of expression appears unintentional
 
 
-==== returns.js (2 errors) ====
+==== returns.js (3 errors) ====
     // @ts-check
     /**
      * @returns {string} This comment is not currently exposed
@@ -22,5 +23,7 @@ returns.js(13,5): error TS2322: Type 'number | boolean' is not assignable to typ
         ~~~~~~
 !!! error TS2322: Type 'number | boolean' is not assignable to type 'string | number'.
 !!! error TS2322:   Type 'boolean' is not assignable to type 'string | number'.
+               ~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
     }
 

tests/baselines/reference/computedPropertyNames46_ES5.errors.txt

@@ -0,0 +1,9 @@
+computedPropertyNames46_ES5.ts(2,6): error TS2869: Truthiness test of this kind of expression appears unintentional
+
+
+==== computedPropertyNames46_ES5.ts (1 errors) ====
+    var o = {
+        ["" || 0]: 0
+         ~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    };

tests/baselines/reference/computedPropertyNames46_ES6.errors.txt

@@ -0,0 +1,9 @@
+computedPropertyNames46_ES6.ts(2,6): error TS2869: Truthiness test of this kind of expression appears unintentional
+
+
+==== computedPropertyNames46_ES6.ts (1 errors) ====
+    var o = {
+        ["" || 0]: 0
+         ~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    };

tests/baselines/reference/computedPropertyNames48_ES5.errors.txt

@@ -0,0 +1,23 @@
+computedPropertyNames48_ES5.ts(16,6): error TS2869: Truthiness test of this kind of expression appears unintentional
+
+
+==== computedPropertyNames48_ES5.ts (1 errors) ====
+    declare function extractIndexer<T>(p: { [n: number]: T }): T;
+
+    enum E { x }
+
+    var a: any;
+
+    extractIndexer({
+        [a]: ""
+    }); // Should return string
+
+    extractIndexer({
+        [E.x]: ""
+    }); // Should return string
+
+    extractIndexer({
+        ["" || 0]: ""
+         ~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    }); // Should return any (widened form of undefined)

tests/baselines/reference/computedPropertyNames48_ES5.types

@@ -17,6 +17,7 @@ enum E { x }
 
 var a: any;
 >a : any
+>  : ^^^
 
 extractIndexer({
 >extractIndexer({    [a]: ""}) : string
@@ -30,6 +31,7 @@ extractIndexer({
 >[a] : string
 >    : ^^^^^^
 >a : any
+>  : ^^^
 >"" : ""
 >   : ^^
 

tests/baselines/reference/computedPropertyNames48_ES6.errors.txt

@@ -0,0 +1,23 @@
+computedPropertyNames48_ES6.ts(16,6): error TS2869: Truthiness test of this kind of expression appears unintentional
+
+
+==== computedPropertyNames48_ES6.ts (1 errors) ====
+    declare function extractIndexer<T>(p: { [n: number]: T }): T;
+
+    enum E { x }
+
+    var a: any;
+
+    extractIndexer({
+        [a]: ""
+    }); // Should return string
+
+    extractIndexer({
+        [E.x]: ""
+    }); // Should return string
+
+    extractIndexer({
+        ["" || 0]: ""
+         ~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    }); // Should return any (widened form of undefined)

tests/baselines/reference/computedPropertyNames48_ES6.types

@@ -17,6 +17,7 @@ enum E { x }
 
 var a: any;
 >a : any
+>  : ^^^
 
 extractIndexer({
 >extractIndexer({    [a]: ""}) : string
@@ -30,6 +31,7 @@ extractIndexer({
 >[a] : string
 >    : ^^^^^^
 >a : any
+>  : ^^^
 >"" : ""
 >   : ^^
 

tests/baselines/reference/conditionalOperatorConditionIsNumberType.errors.txt

@@ -0,0 +1,84 @@
+conditionalOperatorConditionIsNumberType.ts(27,1): error TS2869: Truthiness test of this kind of expression appears unintentional
+conditionalOperatorConditionIsNumberType.ts(29,1): error TS2869: Truthiness test of this kind of expression appears unintentional
+conditionalOperatorConditionIsNumberType.ts(30,1): error TS2869: Truthiness test of this kind of expression appears unintentional
+conditionalOperatorConditionIsNumberType.ts(53,23): error TS2869: Truthiness test of this kind of expression appears unintentional
+conditionalOperatorConditionIsNumberType.ts(55,23): error TS2869: Truthiness test of this kind of expression appears unintentional
+conditionalOperatorConditionIsNumberType.ts(56,32): error TS2869: Truthiness test of this kind of expression appears unintentional
+
+
+==== conditionalOperatorConditionIsNumberType.ts (6 errors) ====
+    //Cond ? Expr1 : Expr2,  Cond is of number type, Expr1 and Expr2 have the same type
+    var condNumber: number;
+
+    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;
+
+    //Cond is a number type variable
+    condNumber ? exprAny1 : exprAny2;
+    condNumber ? exprBoolean1 : exprBoolean2;
+    condNumber ? exprNumber1 : exprNumber2;
+    condNumber ? exprString1 : exprString2;
+    condNumber ? exprIsObject1 : exprIsObject2;
+    condNumber ? exprString1 : exprBoolean1; // Union
+
+    //Cond is a number type literal
+    1 ? exprAny1 : exprAny2;
+    0 ? exprBoolean1 : exprBoolean2;
+    0.123456789 ? exprNumber1 : exprNumber2;
+    ~~~~~~~~~~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    - 10000000000000 ? exprString1 : exprString2;
+    1000000000000 ? exprIsObject1 : exprIsObject2;
+    ~~~~~~~~~~~~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    10000 ? exprString1 : exprBoolean1; // Union
+    ~~~~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+
+    //Cond is a number type expression
+    function foo() { return 1 };
+    var array = [1, 2, 3];
+
+    1 * 0 ? exprAny1 : exprAny2;
+    1 + 1 ? exprBoolean1 : exprBoolean2;
+    "string".length ? exprNumber1 : exprNumber2;
+    foo() ? exprString1 : exprString2;
+    foo() / array[1] ? exprIsObject1 : exprIsObject2;
+    foo() ? exprString1 : exprBoolean1; // Union
+
+    //Results shoud be same as Expr1 and Expr2
+    var resultIsAny1 = condNumber ? exprAny1 : exprAny2;
+    var resultIsBoolean1 = condNumber ? exprBoolean1 : exprBoolean2;
+    var resultIsNumber1 = condNumber ? exprNumber1 : exprNumber2;
+    var resultIsString1 = condNumber ? exprString1 : exprString2;
+    var resultIsObject1 = condNumber ? exprIsObject1 : exprIsObject2;
+    var resultIsStringOrBoolean1 = condNumber ? exprString1 : exprBoolean1; // Union
+
+    var resultIsAny2 = 1 ? exprAny1 : exprAny2;
+    var resultIsBoolean2 = 0 ? exprBoolean1 : exprBoolean2;
+    var resultIsNumber2 = 0.123456789 ? exprNumber1 : exprNumber2;
+                          ~~~~~~~~~~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    var resultIsString2 = - 10000000000000 ? exprString1 : exprString2;
+    var resultIsObject2 = 1000000000000 ? exprIsObject1 : exprIsObject2;
+                          ~~~~~~~~~~~~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+    var resultIsStringOrBoolean2 = 10000 ? exprString1 : exprBoolean1; // Union
+                                   ~~~~~
+!!! error TS2869: Truthiness test of this kind of expression appears unintentional
+
+    var resultIsAny3 = 1 * 0 ? exprAny1 : exprAny2;
+    var resultIsBoolean3 = 1 + 1 ? exprBoolean1 : exprBoolean2;
+    var resultIsNumber3 = "string".length ? exprNumber1 : exprNumber2;
+    var resultIsString3 = foo() ? exprString1 : exprString2;
+    var resultIsObject3 = foo() / array[1] ? exprIsObject1 : exprIsObject2;
+    var resultIsStringOrBoolean3 = foo() / array[1] ? exprString1 : exprBoolean1; // Union