Infer type predicates from function bodies using control flow analysis

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

@@ -15458,9 +15458,19 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         jsdocPredicate = getTypePredicateOfSignature(jsdocSignature);
                     }
                 }
-                signature.resolvedTypePredicate = type && isTypePredicateNode(type) ?
-                    createTypePredicateFromTypePredicateNode(type, signature) :
-                    jsdocPredicate || noTypePredicate;
+                if (type || jsdocPredicate) {
+                    signature.resolvedTypePredicate = type && isTypePredicateNode(type) ?
+                        createTypePredicateFromTypePredicateNode(type, signature) :
+                        jsdocPredicate || noTypePredicate;
+                }
+                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;
+                }
+                else {
+                    signature.resolvedTypePredicate = noTypePredicate;
+                }
             }
             Debug.assert(!!signature.resolvedTypePredicate);
         }
@@ -37389,6 +37399,72 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         }
     }
 
+    function getTypePredicateFromBody(func: FunctionLikeDeclaration): TypePredicate | undefined {
+        switch (func.kind) {
+            case SyntaxKind.Constructor:
+            case SyntaxKind.GetAccessor:
+            case SyntaxKind.SetAccessor:
+                return undefined;
+        }
+        const functionFlags = getFunctionFlags(func);
+        if (functionFlags !== FunctionFlags.Normal || func.parameters.length === 0) return undefined;
+
+        // Only attempt to infer a type predicate if there's exactly one return.
+        let singleReturn: Expression | undefined;
+        if (func.body && func.body.kind !== SyntaxKind.Block) {
+            singleReturn = func.body; // arrow function
+        }
+        else {
+            const bailedEarly = forEachReturnStatement(func.body as Block, returnStatement => {
+                if (singleReturn || !returnStatement.expression) return true;
+                singleReturn = returnStatement.expression;
+            });
+            if (bailedEarly || !singleReturn || functionHasImplicitReturn(func)) return undefined;
+        }
+        return checkIfExpressionRefinesAnyParameter(func, singleReturn);
+    }
+
+    function checkIfExpressionRefinesAnyParameter(func: FunctionLikeDeclaration, expr: Expression): TypePredicate | undefined {
+        expr = skipParentheses(expr, /*excludeJSDocTypeAssertions*/ true);
+        const returnType = checkExpressionCached(expr);
+        if (!(returnType.flags & TypeFlags.Boolean)) return undefined;
+
+        return forEach(func.parameters, (param, i) => {
+            const initType = getTypeOfSymbol(param.symbol);
+            if (!initType || initType.flags & TypeFlags.Boolean || !isIdentifier(param.name) || isSymbolAssigned(param.symbol)) {
+                // Refining "x: boolean" to "x is true" or "x is false" isn't useful.
+                return;
+            }
+            const trueType = checkIfExpressionRefinesParameter(func, expr, param, initType);
+            if (trueType) {
+                return createTypePredicate(TypePredicateKind.Identifier, unescapeLeadingUnderscores(param.name.escapedText), i, trueType);
+            }
+        });
+    }
+
+    function checkIfExpressionRefinesParameter(func: FunctionLikeDeclaration, expr: Expression, param: ParameterDeclaration, initType: Type): Type | undefined {
+        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,
+            antecedent,
+        };
+
+        const trueType = getFlowTypeOfReference(param.name, initType, initType, func, trueCondition);
+        if (trueType === initType) return undefined;
+
+        // "x is T" means that x is T if and only if it returns true. If it returns false then x is not T.
+        // This means that if the function is called with an argument of type trueType, there can't be anything left in the `else` branch. It must reduce to `never`.
+        const falseCondition: FlowCondition = {
+            ...trueCondition,
+            flags: FlowFlags.FalseCondition,
+        };
+        const falseSubtype = getFlowTypeOfReference(param.name, trueType, trueType, func, falseCondition);
+        return falseSubtype.flags & TypeFlags.Never ? trueType : undefined;
+    }
+
     /**
      * TypeScript Specification 1.0 (6.3) - July 2014
      *   An explicitly typed function whose return type isn't the Void type,

tests/baselines/reference/circularConstructorWithReturn.js

@@ -0,0 +1,41 @@
+//// [tests/cases/compiler/circularConstructorWithReturn.ts] ////
+
+//// [circularConstructorWithReturn.ts]
+// This should not be a circularity error. See
+// https://github.com/microsoft/TypeScript/pull/57465#issuecomment-1960271216
+export type Client = ReturnType<typeof getPrismaClient> extends new () => infer T ? T : never
+
+export function getPrismaClient(options?: any) {
+  class PrismaClient {
+    self: Client;
+    constructor(options?: any) {
+      return (this.self = applyModelsAndClientExtensions(this));
+    }
+  }
+
+  return PrismaClient
+}
+
+export function applyModelsAndClientExtensions(client: Client) {
+  return client;
+}
+
+
+//// [circularConstructorWithReturn.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.applyModelsAndClientExtensions = exports.getPrismaClient = void 0;
+function getPrismaClient(options) {
+    var PrismaClient = /** @class */ (function () {
+        function PrismaClient(options) {
+            return (this.self = applyModelsAndClientExtensions(this));
+        }
+        return PrismaClient;
+    }());
+    return PrismaClient;
+}
+exports.getPrismaClient = getPrismaClient;
+function applyModelsAndClientExtensions(client) {
+    return client;
+}
+exports.applyModelsAndClientExtensions = applyModelsAndClientExtensions;

tests/baselines/reference/circularConstructorWithReturn.symbols

@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/circularConstructorWithReturn.ts] ////
+
+=== circularConstructorWithReturn.ts ===
+// This should not be a circularity error. See
+// https://github.com/microsoft/TypeScript/pull/57465#issuecomment-1960271216
+export type Client = ReturnType<typeof getPrismaClient> extends new () => infer T ? T : never
+>Client : Symbol(Client, Decl(circularConstructorWithReturn.ts, 0, 0))
+>ReturnType : Symbol(ReturnType, Decl(lib.es5.d.ts, --, --))
+>getPrismaClient : Symbol(getPrismaClient, Decl(circularConstructorWithReturn.ts, 2, 93))
+>T : Symbol(T, Decl(circularConstructorWithReturn.ts, 2, 79))
+>T : Symbol(T, Decl(circularConstructorWithReturn.ts, 2, 79))
+
+export function getPrismaClient(options?: any) {
+>getPrismaClient : Symbol(getPrismaClient, Decl(circularConstructorWithReturn.ts, 2, 93))
+>options : Symbol(options, Decl(circularConstructorWithReturn.ts, 4, 32))
+
+  class PrismaClient {
+>PrismaClient : Symbol(PrismaClient, Decl(circularConstructorWithReturn.ts, 4, 48))
+
+    self: Client;
+>self : Symbol(PrismaClient.self, Decl(circularConstructorWithReturn.ts, 5, 22))
+>Client : Symbol(Client, Decl(circularConstructorWithReturn.ts, 0, 0))
+
+    constructor(options?: any) {
+>options : Symbol(options, Decl(circularConstructorWithReturn.ts, 7, 16))
+
+      return (this.self = applyModelsAndClientExtensions(this));
+>this.self : Symbol(PrismaClient.self, Decl(circularConstructorWithReturn.ts, 5, 22))
+>this : Symbol(PrismaClient, Decl(circularConstructorWithReturn.ts, 4, 48))
+>self : Symbol(PrismaClient.self, Decl(circularConstructorWithReturn.ts, 5, 22))
+>applyModelsAndClientExtensions : Symbol(applyModelsAndClientExtensions, Decl(circularConstructorWithReturn.ts, 13, 1))
+>this : Symbol(PrismaClient, Decl(circularConstructorWithReturn.ts, 4, 48))
+    }
+  }
+
+  return PrismaClient
+>PrismaClient : Symbol(PrismaClient, Decl(circularConstructorWithReturn.ts, 4, 48))
+}
+
+export function applyModelsAndClientExtensions(client: Client) {
+>applyModelsAndClientExtensions : Symbol(applyModelsAndClientExtensions, Decl(circularConstructorWithReturn.ts, 13, 1))
+>client : Symbol(client, Decl(circularConstructorWithReturn.ts, 15, 47))
+>Client : Symbol(Client, Decl(circularConstructorWithReturn.ts, 0, 0))
+
+  return client;
+>client : Symbol(client, Decl(circularConstructorWithReturn.ts, 15, 47))
+}
+

tests/baselines/reference/circularConstructorWithReturn.types

@@ -0,0 +1,46 @@
+//// [tests/cases/compiler/circularConstructorWithReturn.ts] ////
+
+=== circularConstructorWithReturn.ts ===
+// This should not be a circularity error. See
+// https://github.com/microsoft/TypeScript/pull/57465#issuecomment-1960271216
+export type Client = ReturnType<typeof getPrismaClient> extends new () => infer T ? T : never
+>Client : PrismaClient
+>getPrismaClient : (options?: any) => typeof PrismaClient
+
+export function getPrismaClient(options?: any) {
+>getPrismaClient : (options?: any) => typeof PrismaClient
+>options : any
+
+  class PrismaClient {
+>PrismaClient : PrismaClient
+
+    self: Client;
+>self : PrismaClient
+
+    constructor(options?: any) {
+>options : any
+
+      return (this.self = applyModelsAndClientExtensions(this));
+>(this.self = applyModelsAndClientExtensions(this)) : PrismaClient
+>this.self = applyModelsAndClientExtensions(this) : PrismaClient
+>this.self : PrismaClient
+>this : this
+>self : PrismaClient
+>applyModelsAndClientExtensions(this) : PrismaClient
+>applyModelsAndClientExtensions : (client: PrismaClient) => PrismaClient
+>this : this
+    }
+  }
+
+  return PrismaClient
+>PrismaClient : typeof PrismaClient
+}
+
+export function applyModelsAndClientExtensions(client: Client) {
+>applyModelsAndClientExtensions : (client: Client) => PrismaClient
+>client : PrismaClient
+
+  return client;
+>client : PrismaClient
+}
+