Switch to checking for assignment when inverted

Author: tjenkinson

src/compiler/checker.ts

@@ -44539,30 +44539,30 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     function checkTestingKnownTruthyCallableOrAwaitableOrEnumMemberType(condExpr: Expression, condType: Type, body?: Statement | Expression) {
         if (!strictNullChecks) return;
 
-        bothHelper(condExpr, condType, body);
+        bothHelper(condExpr, body);
 
-        function bothHelper(condExpr: Expression, condType: Type, body: Expression | Statement | undefined) {
+        function bothHelper(condExpr: Expression, body: Expression | Statement | undefined) {
             condExpr = skipParentheses(condExpr);
-            helper(condExpr, condType, body);
+            helper(condExpr, body);
             while (isBinaryExpression(condExpr) && (condExpr.operatorToken.kind === SyntaxKind.BarBarToken || condExpr.operatorToken.kind === SyntaxKind.QuestionQuestionToken)) {
                 condExpr = skipParentheses(condExpr.left);
-                helper(condExpr, condType, body);
+                helper(condExpr, body);
             }
         }
 
-        function helper(condExpr: Expression, condType: Type, body: Expression | Statement | undefined) {
-            let location = condExpr;
-            // let checkAwaitOnly = false;
-            while (isPrefixUnaryExpression(location)) {
-                location = skipParentheses(location.operand);
-                // checkAwaitOnly = true;
+        function helper(condExpr: Expression, body: Expression | Statement | undefined) {
+            let inverted = false;
+            while (isPrefixUnaryExpression(condExpr)) {
+                condExpr = skipParentheses(condExpr.operand);
+                inverted = !inverted;
             }
+            let location = condExpr;
             location = isLogicalOrCoalescingBinaryExpression(location) ? skipParentheses(location.right) : location;
             if (isModuleExportsAccessExpression(location)) {
                 return;
             }
             if (isLogicalOrCoalescingBinaryExpression(location)) {
-                bothHelper(location, condType, body);
+                bothHelper(location, body);
                 return;
             }
             const type = location === condExpr ? condType : checkExpression(location);
@@ -44593,8 +44593,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 return;
             }
 
+            const isUsedInBody = inverted && isIfStatement(condExpr.parent) ? testedSymbol && isSymbolUsedInConditionBody(condExpr, condExpr.parent.parent, testedNode, testedSymbol, /*checkForAssignment*/ true) : testedSymbol && body && isSymbolUsedInConditionBody(condExpr, body, testedNode, testedSymbol, /*checkForAssignment*/ false);
             const isUsed = testedSymbol && isBinaryExpression(condExpr.parent) && isSymbolUsedInBinaryExpressionChain(condExpr.parent, testedSymbol)
-                || testedSymbol && body && isSymbolUsedInConditionBody(condExpr, body, testedNode, testedSymbol);
+                || isUsedInBody;
+
             if (!isUsed) {
                 if (isPromise) {
                     errorAndMaybeSuggestAwait(
@@ -44604,23 +44606,30 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         getTypeNameForErrorDisplay(type),
                     );
                 }
-                // else if (!checkAwaitOnly) {
                 else {
                     error(location, Diagnostics.This_condition_will_always_return_true_since_this_function_is_always_defined_Did_you_mean_to_call_it_instead);
                 }
             }
         }
     }
 
-    function isSymbolUsedInConditionBody(expr: Expression, body: Statement | Expression, testedNode: Node, testedSymbol: Symbol): boolean {
+    function isSymbolUsedInConditionBody(expr: Expression, body: Statement | Expression | Node, testedNode: Node, testedSymbol: Symbol, checkForAssignment: boolean): boolean {
         return !!forEachChild(body, function check(childNode): boolean | undefined {
             if (isIdentifier(childNode)) {
                 const childSymbol = getSymbolAtLocation(childNode);
                 if (childSymbol && childSymbol === testedSymbol) {
-                    // If the test was a simple identifier, the above check is sufficient
-                    if (isIdentifier(expr) || isIdentifier(testedNode) && isBinaryExpression(testedNode.parent)) {
-                        return true;
+                    if (checkForAssignment) {
+                        if (isIdentifier(expr) && isBinaryExpression(childNode.parent) && childNode.parent.operatorToken.kind === SyntaxKind.EqualsToken) {
+                            return true;
+                        }
                     }
+                    else {
+                        // If the test was a simple identifier, the above check is sufficient
+                        if (isIdentifier(expr) || isIdentifier(testedNode) && isBinaryExpression(testedNode.parent)) {
+                            return true;
+                        }
+                    }
+
                     // Otherwise we need to ensure the symbol is called on the same target
                     let testedExpression = testedNode.parent;
                     let childExpression = childNode.parent;
@@ -44629,7 +44638,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                             isIdentifier(testedExpression) && isIdentifier(childExpression) ||
                             testedExpression.kind === SyntaxKind.ThisKeyword && childExpression.kind === SyntaxKind.ThisKeyword
                         ) {
-                            return getSymbolAtLocation(testedExpression) === getSymbolAtLocation(childExpression);
+                            const sameSymbol = getSymbolAtLocation(testedExpression) === getSymbolAtLocation(childExpression);
+                            if (sameSymbol) {
+                                if (checkForAssignment) {
+                                    return isPropertyAccessExpression(childNode.parent) && isBinaryExpression(childExpression.parent.parent) && childExpression.parent.parent.operatorToken.kind === SyntaxKind.EqualsToken;
+                                }
+                                return true;
+                            }
+                            return false;
                         }
                         else if (isPropertyAccessExpression(testedExpression) && isPropertyAccessExpression(childExpression)) {
                             if (getSymbolAtLocation(testedExpression.name) !== getSymbolAtLocation(childExpression.name)) {

tests/baselines/reference/contextualOverloadListFromArrayUnion.errors.txt

@@ -1,5 +1,4 @@
 truthinessCallExpressionCoercion.ts(2,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
-truthinessCallExpressionCoercion.ts(8,11): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion.ts(18,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion.ts(36,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion.ts(50,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
@@ -8,7 +7,7 @@ truthinessCallExpressionCoercion.ts(76,9): error TS2774: This condition will alw
 truthinessCallExpressionCoercion.ts(82,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 
 
-==== truthinessCallExpressionCoercion.ts (8 errors) ====
+==== truthinessCallExpressionCoercion.ts (7 errors) ====
     function onlyErrorsWhenTestingNonNullableFunctionType(required: () => boolean, optional?: () => boolean) {
         if (required) { // error
             ~~~~~~~~
@@ -19,8 +18,6 @@ truthinessCallExpressionCoercion.ts(82,9): error TS2774: This condition will alw
         }
 
         if (!!required) { // ok
-              ~~~~~~~~
-!!! error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
         }
 
         if (required()) { // ok

tests/baselines/reference/truthinessCallExpressionCoercion.errors.txt

@@ -1,12 +1,11 @@
 truthinessCallExpressionCoercion1.ts(3,5): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
-truthinessCallExpressionCoercion1.ts(9,7): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion1.ts(19,5): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion1.ts(33,5): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion1.ts(46,5): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion1.ts(76,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 
 
-==== truthinessCallExpressionCoercion1.ts (6 errors) ====
+==== truthinessCallExpressionCoercion1.ts (5 errors) ====
     function onlyErrorsWhenTestingNonNullableFunctionType(required: () => boolean, optional?: () => boolean) {
         // error
         required ? console.log('required') : undefined;
@@ -18,8 +17,6 @@ truthinessCallExpressionCoercion1.ts(76,9): error TS2774: This condition will al
 
         // ok
         !!required ? console.log('not required') : undefined;
-          ~~~~~~~~
-!!! error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 
         // ok
         required() ? console.log('required call') : undefined;

tests/baselines/reference/truthinessCallExpressionCoercion1.errors.txt

@@ -1,6 +1,5 @@
 truthinessCallExpressionCoercion2.ts(11,5): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion2.ts(14,10): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
-truthinessCallExpressionCoercion2.ts(29,7): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion2.ts(41,18): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion2.ts(44,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 truthinessCallExpressionCoercion2.ts(48,9): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
@@ -36,7 +35,7 @@ truthinessCallExpressionCoercion2.ts(180,9): error TS2774: This condition will a
 truthinessCallExpressionCoercion2.ts(183,14): error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 
 
-==== truthinessCallExpressionCoercion2.ts (36 errors) ====
+==== truthinessCallExpressionCoercion2.ts (35 errors) ====
     declare class A {
         static from(): string;
     }
@@ -70,8 +69,6 @@ truthinessCallExpressionCoercion2.ts(183,14): error TS2774: This condition will
 
         // ok
         !!required1 && console.log('not required');
-          ~~~~~~~~~
-!!! error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
 
         // ok
         required1() && console.log('required call');

tests/baselines/reference/truthinessCallExpressionCoercion2.errors.txt

@@ -1,13 +1,11 @@
 truthinessPromiseCoercion.ts(7,9): error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
-truthinessPromiseCoercion.ts(8,11): error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
 truthinessPromiseCoercion.ts(11,5): error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
-truthinessPromiseCoercion.ts(12,7): error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
 truthinessPromiseCoercion.ts(32,9): error TS2801: This condition will always return true since this 'Promise<unknown>' is always defined.
 truthinessPromiseCoercion.ts(40,9): error TS2801: This condition will always return true since this 'Promise<boolean>' is always defined.
 truthinessPromiseCoercion.ts(43,9): error TS2801: This condition will always return true since this 'Promise<boolean>' is always defined.
 
 
-==== truthinessPromiseCoercion.ts (7 errors) ====
+==== truthinessPromiseCoercion.ts (5 errors) ====
     declare const p: Promise<number>
     declare const p2: null | Promise<number>
     declare const obj: { p: Promise<unknown> }
@@ -19,19 +17,13 @@ truthinessPromiseCoercion.ts(43,9): error TS2801: This condition will always ret
 !!! error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
 !!! related TS2773 truthinessPromiseCoercion.ts:7:9: Did you forget to use 'await'?
         if (!!p) {} // no err
-              ~
-!!! error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
-!!! related TS2773 truthinessPromiseCoercion.ts:8:11: Did you forget to use 'await'?
         if (p2) {} // no err
 
         p ? f.arguments : f.arguments;
         ~
 !!! error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
 !!! related TS2773 truthinessPromiseCoercion.ts:11:5: Did you forget to use 'await'?
         !!p ? f.arguments : f.arguments;
-          ~
-!!! error TS2801: This condition will always return true since this 'Promise<number>' is always defined.
-!!! related TS2773 truthinessPromiseCoercion.ts:12:7: Did you forget to use 'await'?
         p2 ? f.arguments : f.arguments;
     }