Despite being unimplementable, the type definition below does not trigger any warnings from the compiler.
// No type error
type impossible = 0 & string[] & 'anything'
An item cannot simultaneously be a number, a string[], and a string literal.
According to an explanation by Anders Hejlsberg on the topic:
Primitive types can be intersected (e.g.
string & number), but it is not possible to actually create values of such types except forundefined. This limitation arises because these types can be generated lazily through instantiation of generic types, making it challenging to consistently detect and report errors relating to operations that involve creating these types.
In a comment made on July 3, 2015, he elaborated further:
An intersection type allows constituent types to serve as type parameters. For example,
declare function combine<T, U>(obj1: T, obj2: U): T & U;This enhancement provides additional expressiveness, enabling better representation of existing JavaScript practices. However, due to the ability to instantiate type parameters with any type arguments, it is not feasible to pre-check the validity of an intersection type. Therefore, we must be prepared to intersect various types and handle situations where no values are valid (e.g.
string & number). While some may argue that we should raise errors upon instantiation of such types, the deferred nature of type instantiation in a polymorphic system aims to prevent type proliferation and ensures error reporting remains manageable and deterministic.
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