String literal types from typeof is reduced to string when passing through a generic function that "boxes" the type

[benjajaja]

TypeScript Version: 2.7.2

Search Terms: string literal, typeof, generic, structuring, boxing

Code

type StringLiteral = 'A' | 'B';
const map = (genders: StringLiteral[]): { gender: StringLiteral }[] =>
    genders.map(gender => ({ gender }));
// OK

const CONST_A = 'A';
const CONST_B = 'B';
type StringLiteralFromTypeof = typeof CONST_A | typeof CONST_B;

// StringLiteral and StringLiteralFromTypeof have the same type, when you hover them.

const map_typeof_no_generics_struct = (gender: StringLiteralFromTypeof): { gender: StringLiteralFromTypeof } =>
    ({ gender });
// OK if we struct (or "box") but don't pass through a generic function

const map_typeof_map_struct = (genders: StringLiteralFromTypeof[]): { gender: StringLiteralFromTypeof }[] =>
    genders.map(gender => ({ gender }));
// Error: inferred type is { gender: string }[]

const map_typeof_map_nonstruct = (genders: StringLiteralFromTypeof[]): StringLiteralFromTypeof[] =>
    genders.map(gender => gender);
// OK: if not structured into object or array, the type is inferred correctly

const identity = <T>(v: T) => v;
const map_typeof_any_generic = (gender: StringLiteralFromTypeof): { gender: StringLiteralFromTypeof } =>
    identity({ gender });
// Error: inferred type is { gender: string }[]

const map_typeof_any_generic_nonstruct = (gender: StringLiteralFromTypeof): StringLiteralFromTypeof =>
    identity(gender);
// OK: if not structured into object or array, the type is inferred correctly

Expected behavior:
A string literal type that was constructed from typeof someVar | ... should behave the same as a string literal type defined from string values 'val' | ....

Actual behavior:
When both passing through a generic function, AND structuring or "boxing" the type into an object or array, then the type is reduced to string.

Playground Link: link to see inferred types

Related Issues:
unknown

[RyanCavanaugh]

StringLiteral and StringLiteralFromTypeof are not identical, even though they have the same display text. See #22211

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.