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IsLiteral*: Fix behavior with unions - #1482
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July 9, 2026 10:44
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The new union behavior makes sense, but this regresses branded broad primitives: type Brand = {readonly __brand: unique symbol};
type A = IsNumericLiteral<number & Brand>;
//=> true, should be false
type B = IsLiteral<Opaque<number, 'NumberTag'>>;
//=> true, should be falseThe old implementation explicitly stripped branding before checking whether a primitive was broad or literal, so I think the fix is to keep teh new distributive union behavior but reuse that branded-primitive check for numeric/boolean/symbol. |
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Added a new internal type Brand = {readonly __brand: unique symbol};
type A = UnwrapBrand<string & Brand>;
//=> string
type B = UnwrapBrand<Tagged<number, 'Tag'>>;
//=> number
type C = UnwrapBrand<Opaque<bigint, 'Tag'>>;
//=> bigint
type D = UnwrapBrand<PropertyKey & Brand>;
//=> PropertyKey
type E = UnwrapBrand<(1 | 200n | 'foo' | 'bar' | Uppercase<string>) & Brand>;
//=> 1 | 200n | 'foo' | 'bar' | Uppercase<string>Notice that |
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This PR primarily fixes the behavior of
IsLiteral*types when instantiated with a union of literals and non-literals. For example,IsNumericLiteral<1 | bigint>currently returnsfalse, instead it should returnboolean.This PR also makes a couple of other things:
Tagged/LiteralUniontypes.Also, the input to these literal types is currently unconstrained, but I think that should be constrained. For example,
IsStringLiteral<T extends string> = ...instead of justIsStringLiteral<T> = ....