microsoft / microsoft/TypeScript
Mapped types shouldn't transform unknown type
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Description
@ahejlsberg Probably #29740 made this regression.
TypeScript Version: 3.4.0-dev.20190207
Search Terms:
Code
type DeepReadonly<T> =
T extends void ? T :
{ readonly [P in keyof T]: DeepReadonly<T[P]>; };
type m = { a: unknown }; // Also unknown[].
type i = DeepReadonly<m>;
Expected behavior:
i is { readonly a: unknown; }.
Actual behavior:
i is { readonly a: {}; }.
Playground Link:
Related Issues:
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Research direction
Reproduce the provided DeepReadonly snippet with TypeScript 3.4.0-dev.20190207 and compare the mapped result for unknown with the expected type. Trace the mapped-type and unknown-type handling; done means unknown remains unknown, including the noted unknown[] case, rather than becoming {}.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 42/100