tests/cases/compiler/typeParameterArgumentEquivalence.ts(4,5): error TS2322: Type '(item: T) => boolean' is not assignable to type '(item: number) => boolean'. Types of parameters 'item' and 'item' are incompatible. Type 'T' is not assignable to type 'number'. tests/cases/compiler/typeParameterArgumentEquivalence.ts(5,5): error TS2322: Type '(item: number) => boolean' is not assignable to type '(item: T) => boolean'. Types of parameters 'item' and 'item' are incompatible. Type 'number' is not assignable to type 'T'. ==== tests/cases/compiler/typeParameterArgumentEquivalence.ts (2 errors) ==== function foo() { var x: (item: number) => boolean; var y: (item: T) => boolean; x = y; // Should be an error ~ !!! error TS2322: Type '(item: T) => boolean' is not assignable to type '(item: number) => boolean'. !!! error TS2322: Types of parameters 'item' and 'item' are incompatible. !!! error TS2322: Type 'T' is not assignable to type 'number'. y = x; // Shound be an error ~ !!! error TS2322: Type '(item: number) => boolean' is not assignable to type '(item: T) => boolean'. !!! error TS2322: Types of parameters 'item' and 'item' are incompatible. !!! error TS2322: Type 'number' is not assignable to type 'T'. }