[bug] TemplateStringsArray is incompatible with literal array type

TypeScript Version: 2.4.0

Code

// A *self-contained* demonstration of the problem follows...
interface SQLQuery<TResult> {
  __result: TResult;
} 
declare function sql(
  literals: ['SELECT id FROM users'],
  ...placeholders: any[]
): SQLQuery<{id: number}>;

declare function querySync<TResult>(q: SQLQuery<TResult>): Array<TResult>;

const values: Array<{id: number}> = querySync(sql`SELECT id FROM users`);

Expected behavior:

Typescript should see that the input string 'SELECT id FROM users' matches the expected literals of ['SELECT id FROM users'] and use the declared function, allowing me to generate an overloaded version of the sql function for each query.

Actual behavior:

I get the error:

src/index.ts(23,50): error TS2345: Argument of type 'TemplateStringsArray' is not assignable to parameter of type '["SELECT id FROM users"]'.
Property '0' is missing in type 'TemplateStringsArray'.

I think this is contingent on #16592.

Actually, I don't know if this will work even with the changes I have in mind. TemplateStringsArray is effectively a ReadonlyArray<string> - but you can't assign a ReadonlyArray to a mutable Array. So at the minimum you'd have to write something like

interface MyCustomType extends TemplateStringsArray {
  0: "SELECT id FROM users"
}

That would be fine. I'm going to be auto-generating these definitions anyway, by parsing the code to see which functions are called. This would be very much a workaround for #16551.

At the moment, that doesn't seem to work either though.

declare function id<T extends string>(list: Array<T>): Array<T>;
declare function readonlyId<T extends string>(list: ReadonlyArray<T>): ReadonlyArray<T>;

type LiteralType = "foo" | "bar";

const array: Array<LiteralType> = [];
const readonlyArray: ReadonlyArray<LiteralType> = [];

const foo: ReadonlyArray<LiteralType> = id(array);
const bar: ReadonlyArray<LiteralType> = readonlyId(array);
//    ~~~
// Type 'ReadonlyArray<string>' is not assignable to type 'ReadonlyArray<LiteralType>'.
//  Type 'string' is not assignable to type 'LiteralType'.const bar: ReadonlyArray<LiteralType>
const baz: ReadonlyArray<LiteralType> = readonlyId(readonlyArray);

Isn't it a bug that generic type information is being lost when ReadonlyArray is assigned to Array?

Ronny Friman (@R00GER)

Ronny Friman (@R00GER) that looks like a separate bug, it should probably have its own issue.

Daniel Rosenwasser (@DanielRosenwasser)

Daniel Rosenwasser (@DanielRosenwasser) can you give an update on next steps here?

There shouldn't be a type TemplateStringsArray, it should be a const string tuple type, so we can write code:

interface SQL<TSA, VS> {
  texts: TSA;
  values: VS;
}

function sql<TSA extends readonly string[], VS extends any[]>(texts:TSA, ...values: VS): SQL<TSA, VS> {
  return { texts, values };
}

// then
let s: SQL<['select * from person where a=', ' and b=', ''], [number, Date]> = sql`select * from person where a=${1} and b=${new Date()}`;

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[bug] TemplateStringsArray is incompatible with literal array type #16552

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[bug] TemplateStringsArray is incompatible with literal array type #16552

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DanielRosenwasser commented on Jun 16, 2017

DanielRosenwasser commented on Jun 16, 2017

ForbesLindesay commented on Jun 19, 2017

pablobirukov commented on Jun 21, 2017

ForbesLindesay commented on Jun 23, 2017

RyanCavanaugh commented on Aug 15, 2018

xialvjun commented on May 16, 2019

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