t.taggedUnion doesn't accept array from 1.5.3 (tuple only)

[benoitv-code]

Since the 1.5.3 update, t.taggedUnion() doesn't accept arrays anymore, only tuples.

Example:

const test1 = t.type({
    version: t.literal('1')
});

const test2 = t.type({
    version: t.literal('2')
});

const allTests = [ test1, test2 ];

// Works
const taggedOK = t.taggedUnion(
    'version',
    [
        test1,
        test2
    ]
);

// Doesn't work
const taggedNotOK = t.taggedUnion(
    'version',
    allTests
);

// Doesn't work either
const taggedNotOKEither = t.taggedUnion(
    'version',
    [
        test1,
        test2
    ].map((v) => t.exact(v))
);

Error:

[ts]
Argument of type '(TypeC<{ version: LiteralC<"1">; }> | TypeC<{ version: LiteralC<"2">; }>)[]' is not assignable to parameter of type '[Tagged<"version", any, any>, Tagged<"version", any, any>, ...Tagged<"version", any, any>[]]'.
  Type '(TypeC<{ version: LiteralC<"1">; }> | TypeC<{ version: LiteralC<"2">; }>)[]' is missing the following properties from type '[Tagged<"version", any, any>, Tagged<"version", any, any>, ...Tagged<"version", any, any>[]]': 0, 1 [2345]

[gcanti]

@kunnix taggedUnion and union are now aligned with tuple and intersection, i.e. you have to prove to the type checker that you are passing a correct number of elements

[benoitv-code]

@gcanti Which would be great if TypeScript had a nice way to hint for tuple types, but it's not the case yet and for now they're typed as variable-lengths arrays when stored in a variable or passed to .map() (see microsoft/TypeScript#10195 and microsoft/TypeScript#16656)

I understand this is an elegant way to deal with typing, but it currently prevents storing a list of types in a variable, or to automatically enclose a list of types inside a t.exact() (so, forces to copy-and-paste code when these patterns were used).

[gcanti]

Maybe a couple of helpers can help

const tuple = <T extends Array<unknown>>(...t: T): T => {
  return t
}

type MakeExact<T extends Array<t.HasProps>> = { [K in keyof T]: T[K] extends t.HasProps ? t.ExactC<T[K]> : never }

const makeExact = <T extends Array<t.HasProps>>(...elements: T): MakeExact<T> => {
  return elements.map(e => t.exact(e)) as any
}

// ok
const TU1 = t.taggedUnion('version', [test1, test2])

const allTests = tuple(test1, test2)

// ok
const TU2 = t.taggedUnion('version', allTests)

// ok
const TU3 = t.taggedUnion('version', makeExact(...allTests))

// ok
const TU4 = t.taggedUnion('version', makeExact(test1, test2))

[benoitv-code]

Looks like a great workaround!

[benoitv-code]

@gcanti You probably saw that already, but it might help: microsoft/TypeScript#29510