[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’.

[RyanCavanaugh]

@DanielRosenwasser can you give an update on next steps here?

[pablobirukov]

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?