Using a union type for a rest parameter type annotation

[myitcv]

According to the spec (see later in this note), the following should be valid:

function DoIt(...v: number[] | string[]): void {
}

but this gives an error:

A rest parameter must be of an array type

I'm guessing this is therefore an error in the spec?

RestParameter:
   ... BindingIdentifier TypeAnnotation_opt

TypeAnnotation:
   : Type

Type:
   UnionOrIntersectionOrPrimaryType
   FunctionType
   ConstructorType

UnionOrIntersectionOrPrimaryType:
   UnionType
   IntersectionOrPrimaryType

UnionType:
   UnionOrIntersectionOrPrimaryType | IntersectionOrPrimaryType

UnionOrIntersectionOrPrimaryType:
   UnionType
   IntersectionOrPrimaryType

IntersectionOrPrimaryType:
   IntersectionType
   PrimaryType

PrimaryType:
   ParenthesizedType
   PredefinedType
   TypeReference
   ObjectType
   ArrayType
   TupleType
   TypeQuery

ArrayType:
   PrimaryType [no LineTerminator here] [ ]

PredefinedType:
   any
   number
   boolean
   string
   symbol
   void

[RyanCavanaugh]

It's a valid grammatical production, but not valid according to the type system. The grammar is about what's syntactically valid, not necessarily semantically valid (the same way that var x: number = 'foo' is allowed by the grammar, but not allowed by the type system).

We perhaps could change the rule from "a rest parameter must be an array type" to "a rest parameter must be an array type or a union type in which all constituents are array types" but that seems not worth adding complexity for, since it's easy enough to add overloads to clarify the intended behavior:

// Either this
function DoIt1(...v: string[]): void;
function DoIt1(...v: number[]): void;
function DoIt1(...v: any[]): void {
}

// Or maybe this is what you meant?
function DoIt2(...v: Array<number|string>): void {
}

[myitcv]

It's a valid grammatical production, but not valid according to the type system

Got it, thanks.

but that seems not worth adding complexity for, since it's easy enough to add overloads to clarify the intended behavior

Agreed.

// Or maybe this is what you meant?

No, you understood me correctly!

[mischkl]

At least in TS 2.0, you can also write this as follows:

function DoIt(...v: (number | string)[]): void {
}

(discovered in the Protractor type definitions)