Account for right operands & fix a weird error message for leftmost nullish literals in checkNullishCoalesceOperands

src/compiler/checker.ts

@@ -39667,24 +39667,64 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 grammarErrorOnNode(right, Diagnostics._0_and_1_operations_cannot_be_mixed_without_parentheses, tokenToString(right.operatorToken.kind), tokenToString(operatorToken.kind));
             }
 
-            const leftTarget = skipOuterExpressions(left, OuterExpressionKinds.All);
-            const nullishSemantics = getSyntacticNullishnessSemantics(leftTarget);
-            if (nullishSemantics !== PredicateSemantics.Sometimes) {
-                if (node.parent.kind === SyntaxKind.BinaryExpression) {
-                    error(leftTarget, Diagnostics.This_binary_expression_is_never_nullish_Are_you_missing_parentheses);
+            checkNullishCoalesceOperandLeft(node);
+            checkNullishCoalesceOperandRight(node);
+        }
+    }
+
+    function checkNullishCoalesceOperandLeft(node: BinaryExpression) {
+        const leftTarget = skipOuterExpressions(node.left, OuterExpressionKinds.All);
+        let nullishSemantics = getSyntacticNullishnessSemantics(leftTarget);
+        if (nullishSemantics & PredicateSemantics.Literal && isLeftmostNullishCoalesceOperand(node)) {
+            return;
+        }
+
+        nullishSemantics = nullishSemantics & ~PredicateSemantics.Literal;
+
+        if (nullishSemantics !== PredicateSemantics.Sometimes) {
+            if (node.parent.kind === SyntaxKind.BinaryExpression) {
+                error(leftTarget, Diagnostics.This_binary_expression_is_never_nullish_Are_you_missing_parentheses);
+            }
+            else {
+                if (nullishSemantics === PredicateSemantics.Always) {
+                    error(leftTarget, Diagnostics.This_expression_is_always_nullish);
                 }
                 else {
-                    if (nullishSemantics === PredicateSemantics.Always) {
-                        error(leftTarget, Diagnostics.This_expression_is_always_nullish);
-                    }
-                    else {
-                        error(leftTarget, Diagnostics.Right_operand_of_is_unreachable_because_the_left_operand_is_never_nullish);
-                    }
+                    error(leftTarget, Diagnostics.Right_operand_of_is_unreachable_because_the_left_operand_is_never_nullish);
                 }
             }
         }
     }
 
+    function checkNullishCoalesceOperandRight(node: BinaryExpression) {
+        if (isRightmostNullishCoalesceOperand(node)) {
+            return;
+        }
+
+        const rightTarget = skipOuterExpressions(node.right, OuterExpressionKinds.All);
+        const nullishSemantics = getSyntacticNullishnessSemantics(rightTarget) & ~PredicateSemantics.Literal;
+        if (nullishSemantics !== PredicateSemantics.Sometimes) {
+            if (nullishSemantics === PredicateSemantics.Always) {
+                error(rightTarget, Diagnostics.This_expression_is_always_nullish);
+            }
+            else {
+                error(rightTarget, Diagnostics.This_expression_is_never_nullish);
+            }
+        }
+    }
+
+    function isLeftmostNullishCoalesceOperand(node: BinaryExpression) {
+        while (isBinaryExpression(node.parent) && node.parent.operatorToken.kind === SyntaxKind.QuestionQuestionToken && node.parent.left === node) {
+            node = node.parent;
+        }
+
+        return !isBinaryExpression(node.parent) || node.parent.operatorToken.kind !== SyntaxKind.QuestionQuestionToken;
+    }
+
+    function isRightmostNullishCoalesceOperand(node: BinaryExpression) {
+        return !isBinaryExpression(node.parent) || node.parent.operatorToken.kind !== SyntaxKind.QuestionQuestionToken;
+    }
+
     function getSyntacticNullishnessSemantics(node: Node): PredicateSemantics {
         node = skipOuterExpressions(node);
         switch (node.kind) {
@@ -39710,12 +39750,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
                 return PredicateSemantics.Never;
             case SyntaxKind.ConditionalExpression:
-                return getSyntacticNullishnessSemantics((node as ConditionalExpression).whenTrue) | getSyntacticNullishnessSemantics((node as ConditionalExpression).whenFalse);
+                return (getSyntacticNullishnessSemantics((node as ConditionalExpression).whenTrue) | getSyntacticNullishnessSemantics((node as ConditionalExpression).whenFalse)) & ~PredicateSemantics.Literal;
             case SyntaxKind.NullKeyword:
-                return PredicateSemantics.Always;
+                return PredicateSemantics.Always | PredicateSemantics.Literal;
             case SyntaxKind.Identifier:
                 if (getResolvedSymbol(node as Identifier) === undefinedSymbol) {
-                    return PredicateSemantics.Always;
+                    return PredicateSemantics.Always | PredicateSemantics.Literal;
                 }
                 return PredicateSemantics.Sometimes;
         }

src/compiler/diagnosticMessages.json

@@ -3940,6 +3940,10 @@
         "category": "Error",
         "code": 2873
     },
+    "This expression is never nullish.": {
+        "category": "Error",
+        "code": 2874
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000

src/compiler/types.ts

@@ -928,6 +928,7 @@ export const enum PredicateSemantics {
     None = 0,
     Always = 1 << 0,
     Never = 1 << 1,
+    Literal = 1 << 2,
     Sometimes = Always | Never,
 }
 

tests/baselines/reference/predicateSemantics.errors.txt

@@ -1,34 +1,42 @@
 predicateSemantics.ts(7,16): error TS2871: This expression is always nullish.
 predicateSemantics.ts(10,16): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
-predicateSemantics.ts(26,12): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
-predicateSemantics.ts(27,12): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
-predicateSemantics.ts(28,12): error TS2871: This expression is always nullish.
-predicateSemantics.ts(29,12): error TS2872: This kind of expression is always truthy.
-predicateSemantics.ts(30,12): error TS2872: This kind of expression is always truthy.
-predicateSemantics.ts(33,8): error TS2872: This kind of expression is always truthy.
-predicateSemantics.ts(34,11): error TS2872: This kind of expression is always truthy.
-predicateSemantics.ts(35,8): error TS2872: This kind of expression is always truthy.
-predicateSemantics.ts(36,8): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(26,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(27,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(30,13): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(31,13): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(32,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(34,20): error TS2871: This expression is always nullish.
+predicateSemantics.ts(35,20): error TS2871: This expression is always nullish.
+predicateSemantics.ts(38,29): error TS2871: This expression is always nullish.
+predicateSemantics.ts(40,22): error TS2871: This expression is always nullish.
+predicateSemantics.ts(41,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(41,29): error TS2871: This expression is always nullish.
+predicateSemantics.ts(42,21): error TS2874: This expression is never nullish.
+predicateSemantics.ts(43,22): error TS2874: This expression is never nullish.
+predicateSemantics.ts(46,8): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(47,11): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(48,8): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(49,8): error TS2872: This kind of expression is always truthy.
 
 
-==== predicateSemantics.ts (11 errors) ====
-    declare let cond: any;
+==== predicateSemantics.ts (19 errors) ====
+    declare let opt: number | undefined;
 
     // OK: One or other operand is possibly nullish
-    const test1 = (cond ? undefined : 32) ?? "possibly reached";
+    const test1 = (opt ? undefined : 32) ?? "possibly reached";
 
     // Not OK: Both operands nullish
-    const test2 = (cond ? undefined : null) ?? "always reached";
-                   ~~~~~~~~~~~~~~~~~~~~~~~
+    const test2 = (opt ? undefined : null) ?? "always reached";
+                   ~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2871: This expression is always nullish.
 
     // Not OK: Both operands non-nullish
-    const test3 = (cond ? 132 : 17) ?? "unreachable";
-                   ~~~~~~~~~~~~~~~
+    const test3 = (opt ? 132 : 17) ?? "unreachable";
+                   ~~~~~~~~~~~~~~
 !!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
 
     // Parens
-    const test4 = (cond ? (undefined) : (17)) ?? 42;
+    const test4 = (opt ? (undefined) : (17)) ?? 42;
 
     // Should be OK (special case)
     if (!!true) {
@@ -41,21 +49,50 @@ predicateSemantics.ts(36,8): error TS2872: This kind of expression is always tru
     while (true) { }
     while (false) { }
 
-    const p5 = {} ?? null;
-               ~~
+    const p01 = {} ?? null;
+                ~~
 !!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
-    const p6 = 0 > 1 ?? null;
-               ~~~~~
+    const p02 = 0 > 1 ?? null;
+                ~~~~~
 !!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
-    const p7 = null ?? null;
-               ~~~~
-!!! error TS2871: This expression is always nullish.
-    const p8 = (class foo { }) && null;
-               ~~~~~~~~~~~~~~~
+    const p03 = null ?? 1;
+    const p04 = null ?? null;
+    const p05 = (class foo { }) && null;
+                ~~~~~~~~~~~~~~~
 !!! error TS2872: This kind of expression is always truthy.
-    const p9 = (class foo { }) || null;
-               ~~~~~~~~~~~~~~~
+    const p06 = (class foo { }) || null;
+                ~~~~~~~~~~~~~~~
 !!! error TS2872: This kind of expression is always truthy.
+    const p07 = null ?? null ?? null;
+                        ~~~~
+!!! error TS2871: This expression is always nullish.
+
+    const p10 = opt ?? null ?? 1;
+                       ~~~~
+!!! error TS2871: This expression is always nullish.
+    const p11 = opt ?? null ?? null;
+                       ~~~~
+!!! error TS2871: This expression is always nullish.
+    const p12 = opt ?? (null ?? 1);
+    const p13 = opt ?? (null ?? null);
+    const p14 = opt ?? (null ?? null ?? null);
+                                ~~~~
+!!! error TS2871: This expression is always nullish.
+
+    const p20 = null ?? (opt ? null : undefined) ?? null;
+                         ~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+    const p21 = null ?? null ?? null ?? null;
+                        ~~~~
+!!! error TS2871: This expression is always nullish.
+                                ~~~~
+!!! error TS2871: This expression is always nullish.
+    const p22 = null ?? 1 ?? 1;
+                        ~
+!!! error TS2874: This expression is never nullish.
+    const p23 = null ?? (opt ? 1 : 2) ?? 1;
+                         ~~~~~~~~~~~
+!!! error TS2874: This expression is never nullish.
 
     // Outer expression tests
     while ({} as any) { }
@@ -71,6 +108,8 @@ predicateSemantics.ts(36,8): error TS2872: This kind of expression is always tru
            ~~~~~~
 !!! error TS2872: This kind of expression is always truthy.
 
+    declare let cond: any;
+
     // Should be OK
     console.log((cond || undefined) && 1 / cond);
 

tests/baselines/reference/predicateSemantics.js

@@ -1,19 +1,19 @@
 //// [tests/cases/compiler/predicateSemantics.ts] ////
 
 //// [predicateSemantics.ts]
-declare let cond: any;
+declare let opt: number | undefined;
 
 // OK: One or other operand is possibly nullish
-const test1 = (cond ? undefined : 32) ?? "possibly reached";
+const test1 = (opt ? undefined : 32) ?? "possibly reached";
 
 // Not OK: Both operands nullish
-const test2 = (cond ? undefined : null) ?? "always reached";
+const test2 = (opt ? undefined : null) ?? "always reached";
 
 // Not OK: Both operands non-nullish
-const test3 = (cond ? 132 : 17) ?? "unreachable";
+const test3 = (opt ? 132 : 17) ?? "unreachable";
 
 // Parens
-const test4 = (cond ? (undefined) : (17)) ?? 42;
+const test4 = (opt ? (undefined) : (17)) ?? 42;
 
 // Should be OK (special case)
 if (!!true) {
@@ -26,32 +26,47 @@ while (1) { }
 while (true) { }
 while (false) { }
 
-const p5 = {} ?? null;
-const p6 = 0 > 1 ?? null;
-const p7 = null ?? null;
-const p8 = (class foo { }) && null;
-const p9 = (class foo { }) || null;
+const p01 = {} ?? null;
+const p02 = 0 > 1 ?? null;
+const p03 = null ?? 1;
+const p04 = null ?? null;
+const p05 = (class foo { }) && null;
+const p06 = (class foo { }) || null;
+const p07 = null ?? null ?? null;
+
+const p10 = opt ?? null ?? 1;
+const p11 = opt ?? null ?? null;
+const p12 = opt ?? (null ?? 1);
+const p13 = opt ?? (null ?? null);
+const p14 = opt ?? (null ?? null ?? null);
+
+const p20 = null ?? (opt ? null : undefined) ?? null;
+const p21 = null ?? null ?? null ?? null;
+const p22 = null ?? 1 ?? 1;
+const p23 = null ?? (opt ? 1 : 2) ?? 1;
 
 // Outer expression tests
 while ({} as any) { }
 while ({} satisfies unknown) { }
 while ((<any>({}))) { }
 while ((({}))) { }
 
+declare let cond: any;
+
 // Should be OK
 console.log((cond || undefined) && 1 / cond);
 
 
 //// [predicateSemantics.js]
-var _a, _b, _c, _d, _e, _f;
+var _a, _b, _c, _d, _e, _f, _g, _h, _j, _k, _l, _m, _o, _p, _q;
 // OK: One or other operand is possibly nullish
-var test1 = (_a = (cond ? undefined : 32)) !== null && _a !== void 0 ? _a : "possibly reached";
+var test1 = (_a = (opt ? undefined : 32)) !== null && _a !== void 0 ? _a : "possibly reached";
 // Not OK: Both operands nullish
-var test2 = (_b = (cond ? undefined : null)) !== null && _b !== void 0 ? _b : "always reached";
+var test2 = (_b = (opt ? undefined : null)) !== null && _b !== void 0 ? _b : "always reached";
 // Not OK: Both operands non-nullish
-var test3 = (_c = (cond ? 132 : 17)) !== null && _c !== void 0 ? _c : "unreachable";
+var test3 = (_c = (opt ? 132 : 17)) !== null && _c !== void 0 ? _c : "unreachable";
 // Parens
-var test4 = (_d = (cond ? (undefined) : (17))) !== null && _d !== void 0 ? _d : 42;
+var test4 = (_d = (opt ? (undefined) : (17))) !== null && _d !== void 0 ? _d : 42;
 // Should be OK (special case)
 if (!!true) {
 }
@@ -60,19 +75,30 @@ while (0) { }
 while (1) { }
 while (true) { }
 while (false) { }
-var p5 = (_e = {}) !== null && _e !== void 0 ? _e : null;
-var p6 = (_f = 0 > 1) !== null && _f !== void 0 ? _f : null;
-var p7 = null !== null && null !== void 0 ? null : null;
-var p8 = (/** @class */ (function () {
+var p01 = (_e = {}) !== null && _e !== void 0 ? _e : null;
+var p02 = (_f = 0 > 1) !== null && _f !== void 0 ? _f : null;
+var p03 = null !== null && null !== void 0 ? null : 1;
+var p04 = null !== null && null !== void 0 ? null : null;
+var p05 = (/** @class */ (function () {
     function foo() {
     }
     return foo;
 }())) && null;
-var p9 = (/** @class */ (function () {
+var p06 = (/** @class */ (function () {
     function foo() {
     }
     return foo;
 }())) || null;
+var p07 = (_g = null !== null && null !== void 0 ? null : null) !== null && _g !== void 0 ? _g : null;
+var p10 = (_h = opt !== null && opt !== void 0 ? opt : null) !== null && _h !== void 0 ? _h : 1;
+var p11 = (_j = opt !== null && opt !== void 0 ? opt : null) !== null && _j !== void 0 ? _j : null;
+var p12 = opt !== null && opt !== void 0 ? opt : (null !== null && null !== void 0 ? null : 1);
+var p13 = opt !== null && opt !== void 0 ? opt : (null !== null && null !== void 0 ? null : null);
+var p14 = opt !== null && opt !== void 0 ? opt : ((_k = null !== null && null !== void 0 ? null : null) !== null && _k !== void 0 ? _k : null);
+var p20 = (_l = null !== null && null !== void 0 ? null : (opt ? null : undefined)) !== null && _l !== void 0 ? _l : null;
+var p21 = (_o = (_m = null !== null && null !== void 0 ? null : null) !== null && _m !== void 0 ? _m : null) !== null && _o !== void 0 ? _o : null;
+var p22 = (_p = null !== null && null !== void 0 ? null : 1) !== null && _p !== void 0 ? _p : 1;
+var p23 = (_q = null !== null && null !== void 0 ? null : (opt ? 1 : 2)) !== null && _q !== void 0 ? _q : 1;
 // Outer expression tests
 while ({}) { }
 while ({}) { }