Merge branch 'suggested-changes-57465' into infer-type-predicate-16069

src/compiler/binder.ts

@@ -1094,7 +1094,7 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
             inAssignmentPattern = saveInAssignmentPattern;
             return;
         }
-        if (node.kind >= SyntaxKind.FirstStatement && node.kind <= SyntaxKind.LastStatement && !options.allowUnreachableCode) {
+        if (node.kind >= SyntaxKind.FirstStatement && node.kind <= SyntaxKind.LastStatement && (!options.allowUnreachableCode || node.kind === SyntaxKind.ReturnStatement)) {
             (node as HasFlowNode).flowNode = currentFlow;
         }
         switch (node.kind) {

src/compiler/checker.ts

@@ -15463,7 +15463,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         createTypePredicateFromTypePredicateNode(type, signature) :
                         jsdocPredicate || noTypePredicate;
                 }
-                else if (signature.declaration && isFunctionLikeDeclaration(signature.declaration) && (!signature.resolvedReturnType || signature.resolvedReturnType === booleanType)) {
+                else if (signature.declaration && isFunctionLikeDeclaration(signature.declaration) && (!signature.resolvedReturnType || signature.resolvedReturnType.flags & TypeFlags.Boolean)) {
                     const { declaration } = signature;
                     signature.resolvedTypePredicate = noTypePredicate; // avoid infinite loop
                     signature.resolvedTypePredicate = getTypePredicateFromBody(declaration) || noTypePredicate;
@@ -37411,7 +37411,6 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
         // Only attempt to infer a type predicate if there's exactly one return.
         let singleReturn: Expression | undefined;
-        let singleReturnStatement: ReturnStatement | undefined;
         if (func.body && func.body.kind !== SyntaxKind.Block) {
             singleReturn = func.body; // arrow function
         }
@@ -37420,7 +37419,6 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
             const bailedEarly = forEachReturnStatement(func.body as Block, returnStatement => {
                 if (singleReturn || !returnStatement.expression) return true;
-                singleReturnStatement = returnStatement;
                 singleReturn = returnStatement.expression;
             });
             if (bailedEarly || !singleReturn) return undefined;
@@ -37439,11 +37437,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         function checkIfExpressionRefinesAnyParameter(expr: Expression): [number, Type] | undefined {
             expr = skipParentheses(expr, /*excludeJSDocTypeAssertions*/ true);
             const type = checkExpressionCached(expr);
-            if (type !== booleanType) return undefined;
+            if (!(type.flags & TypeFlags.Boolean)) return undefined;
 
             return forEach(func.parameters, (param, i) => {
-                const initType = getSymbolLinks(param.symbol).type;
-                if (!initType || initType === booleanType || isSymbolAssigned(param.symbol)) {
+                const initType = getTypeOfSymbol(param.symbol);
+                if (!initType || initType.flags & TypeFlags.Boolean || isSymbolAssigned(param.symbol)) {
                     // Refining "x: boolean" to "x is true" or "x is false" isn't useful.
                     return;
                 }
@@ -37455,7 +37453,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         }
 
         function checkIfExpressionRefinesParameter(expr: Expression, param: ParameterDeclaration, initType: Type): Type | undefined {
-            const antecedent = (expr as Expression & { flowNode?: FlowNode; }).flowNode ?? { flags: FlowFlags.Start };
+            const antecedent = (expr as Expression & { flowNode?: FlowNode; }).flowNode ||
+                expr.parent.kind === SyntaxKind.ReturnStatement && (expr.parent as ReturnStatement).flowNode ||
+                { flags: FlowFlags.Start };
             const trueCondition: FlowCondition = {
                 flags: FlowFlags.TrueCondition,
                 node: expr,
@@ -37472,19 +37472,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 flags: FlowFlags.FalseCondition,
             };
             const falseSubtype = getFlowTypeOfReference(param.name, trueType, trueType, func, falseCondition);
-            if (!isTypeIdenticalTo(falseSubtype, neverType)) return undefined;
-
-            // the parameter type may already have been narrowed due to an assertion.
-            // There's no precise way to represent an assertion that's also a predicate. Best not to try.
-            // We do this check last since it's unlikely to filter out many possible predicates.
-            if (singleReturnStatement?.flowNode) {
-                const typeAtReturn = getFlowTypeOfReference(param.name, initType, initType, func, singleReturnStatement?.flowNode);
-                if (typeAtReturn !== initType) {
-                    return undefined;
-                }
-            }
-
-            return trueType;
+            return falseSubtype.flags & TypeFlags.Never ? trueType : undefined;
         }
     }
 

tests/baselines/reference/inferTypePredicates.errors.txt

@@ -14,11 +14,9 @@ inferTypePredicates.ts(113,7): error TS2322: Type 'string | number' is not assig
 inferTypePredicates.ts(115,7): error TS2322: Type 'string | number' is not assignable to type 'number'.
   Type 'string' is not assignable to type 'number'.
 inferTypePredicates.ts(205,7): error TS2741: Property 'z' is missing in type 'C1' but required in type 'C2'.
-inferTypePredicates.ts(252,7): error TS2322: Type 'string | number | Date' is not assignable to type 'string'.
-  Type 'number' is not assignable to type 'string'.
 
 
-==== inferTypePredicates.ts (11 errors) ====
+==== inferTypePredicates.ts (10 errors) ====
     // https://github.com/microsoft/TypeScript/issues/16069
     
     const numsOrNull = [1, 2, 3, 4, null];
@@ -298,9 +296,6 @@ inferTypePredicates.ts(252,7): error TS2322: Type 'string | number | Date' is no
     declare let snd: string | number | Date;
     if (assertAndPredicate(snd)) {
       let t: string = snd; // should error
-          ~
-!!! error TS2322: Type 'string | number | Date' is not assignable to type 'string'.
-!!! error TS2322:   Type 'number' is not assignable to type 'string'.
     }
     
     function isNumberWithThis(this: Date, x: number | string) {

tests/baselines/reference/inferTypePredicates.types

@@ -886,7 +886,7 @@ if (isNumOrStr(unk)) {
 
 // A function can be a type predicate even if it throws.
 function assertAndPredicate(x: string | number | Date) {
->assertAndPredicate : (x: string | number | Date) => boolean
+>assertAndPredicate : (x: string | number | Date) => x is string
 >x : string | number | Date
 
   if (x instanceof Date) {
@@ -910,12 +910,12 @@ declare let snd: string | number | Date;
 
 if (assertAndPredicate(snd)) {
 >assertAndPredicate(snd) : boolean
->assertAndPredicate : (x: string | number | Date) => boolean
+>assertAndPredicate : (x: string | number | Date) => x is string
 >snd : string | number | Date
 
   let t: string = snd; // should error
 >t : string
->snd : string | number | Date
+>snd : string
 }
 
 function isNumberWithThis(this: Date, x: number | string) {