tests/cases/compiler/typeParameterArgumentEquivalence5.ts(4,5): error TS2322: Type '() => (item: any) => T' is not assignable to type '() => (item: any) => U'. Type '(item: any) => T' is not assignable to type '(item: any) => U'. Type 'T' is not assignable to type 'U'. tests/cases/compiler/typeParameterArgumentEquivalence5.ts(5,5): error TS2322: Type '() => (item: any) => U' is not assignable to type '() => (item: any) => T'. Type '(item: any) => U' is not assignable to type '(item: any) => T'. Type 'U' is not assignable to type 'T'. ==== tests/cases/compiler/typeParameterArgumentEquivalence5.ts (2 errors) ==== function foo() { var x: () => (item) => U; var y: () => (item) => T; x = y; // Should be an error ~ !!! error TS2322: Type '() => (item: any) => T' is not assignable to type '() => (item: any) => U'. !!! error TS2322: Type '(item: any) => T' is not assignable to type '(item: any) => U'. !!! error TS2322: Type 'T' is not assignable to type 'U'. y = x; // Shound be an error ~ !!! error TS2322: Type '() => (item: any) => U' is not assignable to type '() => (item: any) => T'. !!! error TS2322: Type '(item: any) => U' is not assignable to type '(item: any) => T'. !!! error TS2322: Type 'U' is not assignable to type 'T'. }