tests/cases/compiler/lambdaParamTypes.ts(17,31): error TS2339: Property 'foo' does not exist on type '{ name: string; id: number; }'. tests/cases/compiler/lambdaParamTypes.ts(18,31): error TS2339: Property 'foo' does not exist on type '{ name: string; id: number; }'. tests/cases/compiler/lambdaParamTypes.ts(19,34): error TS2339: Property 'charAt' does not exist on type 'number'. tests/cases/compiler/lambdaParamTypes.ts(20,36): error TS2339: Property 'toExponential' does not exist on type 'string'. tests/cases/compiler/lambdaParamTypes.ts(21,34): error TS2339: Property 'charAt' does not exist on type 'number'. tests/cases/compiler/lambdaParamTypes.ts(22,36): error TS2339: Property 'toExponential' does not exist on type 'string'. ==== tests/cases/compiler/lambdaParamTypes.ts (6 errors) ==== interface MyArrayWrapper { constructor(initialItems?: T[]); doSomething(predicate: (x: T, y: T) => string): void; } declare function create(initialValues?: T[]): MyArrayWrapper; var thing = create([{ name: "bob", id: 24 }, { name: "doug", id: 32 }]); // Below should all be OK thing.doSomething((x, y) => x.name.charAt(0)); // x.name should be string, so should be OK thing.doSomething((x, y) => x.id.toExponential(0)); // x.id should be string, so should be OK thing.doSomething((x, y) => y.name.charAt(0)); // x.name should be string, so should be OK thing.doSomething((x, y) => y.id.toExponential(0)); // x.id should be string, so should be OK // Below should all be in error thing.doSomething((x, y) => x.foo); // no such property on x ~~~ !!! error TS2339: Property 'foo' does not exist on type '{ name: string; id: number; }'. thing.doSomething((x, y) => y.foo); // no such property on y ~~~ !!! error TS2339: Property 'foo' does not exist on type '{ name: string; id: number; }'. thing.doSomething((x, y) => x.id.charAt(0)); // x.id should be number, no charAt member ~~~~~~ !!! error TS2339: Property 'charAt' does not exist on type 'number'. thing.doSomething((x, y) => x.name.toExponential(0)); // x.name should be string, no toExponential member ~~~~~~~~~~~~~ !!! error TS2339: Property 'toExponential' does not exist on type 'string'. thing.doSomething((x, y) => y.id.charAt(0)); ~~~~~~ !!! error TS2339: Property 'charAt' does not exist on type 'number'. thing.doSomething((x, y) => y.name.toExponential(0)); ~~~~~~~~~~~~~ !!! error TS2339: Property 'toExponential' does not exist on type 'string'.