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

src/compiler/checker.ts

@@ -39667,24 +39667,67 @@ 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);
-                }
-                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);
-                    }
+            const leftNullishSemantics = checkNullishCoalesceOperandLeft(node);
+            const rightNullishSemantics = checkNullishCoalesceOperandRight(node);
+
+            // check self if not checked by left operand of parent nullish coalesce expression
+            if (leftNullishSemantics === PredicateSemantics.Always && isNotWithinNullishCoalesceExpression(node)) {
+                if (rightNullishSemantics === PredicateSemantics.Always) {
+                    error(node, Diagnostics.This_expression_is_always_nullish);
                 }
             }
         }
     }
 
+    function checkNullishCoalesceOperandLeft(node: BinaryExpression) {
+        const leftTarget = skipOuterExpressions(node.left, OuterExpressionKinds.All);
+
+        let nullishSemantics = getSyntacticNullishnessSemantics(leftTarget);
+        const isLiteral = nullishSemantics & PredicateSemantics.Literal;
+        nullishSemantics = nullishSemantics & ~PredicateSemantics.Literal;
+
+        if (isLiteral && isLeftmostNullishCoalesceOperand(node)) {
+            return nullishSemantics;
+        }
+
+        if (nullishSemantics !== PredicateSemantics.Sometimes) {
+            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);
+            }
+        }
+
+        return nullishSemantics;
+    }
+
+    function checkNullishCoalesceOperandRight(node: BinaryExpression) {
+        const rightTarget = skipOuterExpressions(node.right, OuterExpressionKinds.All);
+        const nullishSemantics = getSyntacticNullishnessSemantics(rightTarget) & ~PredicateSemantics.Literal;
+        if (isNotWithinNullishCoalesceExpression(node)) {
+            return nullishSemantics;
+        }
+
+        if (nullishSemantics === PredicateSemantics.Always) {
+            error(rightTarget, Diagnostics.This_expression_is_always_nullish);
+        }
+
+        return nullishSemantics;
+    }
+
+    function isLeftmostNullishCoalesceOperand(node: BinaryExpression) {
+        while (isBinaryExpression(node.parent) && node.parent.operatorToken.kind === SyntaxKind.QuestionQuestionToken && node.parent.left === node) {
+            node = node.parent;
+        }
+
+        return isNotWithinNullishCoalesceExpression(node);
+    }
+
+    function isNotWithinNullishCoalesceExpression(node: BinaryExpression) {
+        return !isBinaryExpression(node.parent) || node.parent.operatorToken.kind !== SyntaxKind.QuestionQuestionToken;
+    }
+
     function getSyntacticNullishnessSemantics(node: Node): PredicateSemantics {
         node = skipOuterExpressions(node);
         switch (node.kind) {
@@ -39698,24 +39741,35 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             case SyntaxKind.BinaryExpression:
                 // List of operators that can produce null/undefined:
                 // = ??= ?? || ||= && &&=
-                switch ((node as BinaryExpression).operatorToken.kind) {
-                    case SyntaxKind.EqualsToken:
+                const binaryExpression = node as BinaryExpression;
+                if (binaryExpression.operatorToken.kind === SyntaxKind.EqualsToken) {
+                    return getSyntacticNullishnessSemantics(binaryExpression.right) & ~PredicateSemantics.Literal;
+                }
+
+                const leftSemantics = getSyntacticNullishnessSemantics(binaryExpression.left) & ~PredicateSemantics.Literal;
+                const rightSemantics = getSyntacticNullishnessSemantics(binaryExpression.right) & ~PredicateSemantics.Literal;
+                switch (binaryExpression.operatorToken.kind) {
                     case SyntaxKind.QuestionQuestionToken:
                     case SyntaxKind.QuestionQuestionEqualsToken:
                     case SyntaxKind.BarBarToken:
                     case SyntaxKind.BarBarEqualsToken:
+                        return leftSemantics === PredicateSemantics.Never || rightSemantics === PredicateSemantics.Never ? PredicateSemantics.Never :
+                            leftSemantics === PredicateSemantics.Sometimes || rightSemantics === PredicateSemantics.Sometimes ? PredicateSemantics.Sometimes :
+                            PredicateSemantics.Always;
                     case SyntaxKind.AmpersandAmpersandToken:
                     case SyntaxKind.AmpersandAmpersandEqualsToken:
-                        return PredicateSemantics.Sometimes;
+                        return leftSemantics === PredicateSemantics.Never && rightSemantics === PredicateSemantics.Never ? PredicateSemantics.Never :
+                            leftSemantics === PredicateSemantics.Sometimes && rightSemantics === PredicateSemantics.Sometimes ? PredicateSemantics.Sometimes :
+                            PredicateSemantics.Always;
                 }
                 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 +1,56 @@
 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(29,13): error TS2871: This expression is always 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(32,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(32,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(32,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(34,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(34,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(34,22): error TS2871: This expression is always nullish.
+predicateSemantics.ts(36,20): error TS2871: This expression is always nullish.
+predicateSemantics.ts(37,20): error TS2871: This expression is always nullish.
+predicateSemantics.ts(39,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(40,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(40,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(40,29): error TS2871: This expression is always nullish.
+predicateSemantics.ts(41,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(42,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(43,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(45,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(45,29): error TS2871: This expression is always nullish.
+predicateSemantics.ts(46,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(47,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(50,8): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(51,11): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(52,8): error TS2872: This kind of expression is always truthy.
+predicateSemantics.ts(53,8): error TS2872: This kind of expression is always truthy.
 
 
-==== predicateSemantics.ts (11 errors) ====
-    declare let cond: any;
+==== predicateSemantics.ts (33 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 +63,82 @@ 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;
-               ~~~~
+    const p03 = null ?? 1;
+    const p04 = null ?? null;
+                ~~~~~~~~~~~~
 !!! error TS2871: This expression is always nullish.
-    const p8 = (class foo { }) && 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.
+                ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                        ~~~~
+!!! error TS2871: This expression is always nullish.
+    const p08 = null ?? opt ?? null;
+    const p09 = null ?? (opt ? null : undefined) ?? null;
+                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                         ~~~~~~~~~~~~~~~~~~~~~~
+!!! 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);
+                        ~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+    const p14 = opt ?? (null ?? null ?? null);
+                        ~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                        ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                                ~~~~
+!!! error TS2871: This expression is always nullish.
+    const p15 = opt ?? (opt ? null : undefined) ?? null;
+                        ~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+    const p16 = opt ?? 1 ?? 2;
+                ~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+    const p17 = opt ?? (opt ? 1 : 2) ?? 3;
+                ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+
+    const p21 = null ?? null ?? null ?? null;
+                ~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2871: This expression is always nullish.
+                        ~~~~
+!!! error TS2871: This expression is always nullish.
+                                ~~~~
+!!! error TS2871: This expression is always nullish.
+    const p22 = null ?? 1 ?? 1;
+                ~~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+    const p23 = null ?? (opt ? 1 : 2) ?? 1;
+                ~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
 
     // Outer expression tests
     while ({} as any) { }
@@ -71,6 +154,8 @@ 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);