{} and { [K in never]: any } exhibit different simplification behavior

Bug Report

Under some circumstances, TypeScript treats { [K in never]: any } & { foo?: number } differently than {} & { foo?: number }. However, this does not occur with { [K in never]: any } & { foo: number }.

🔎 Search Terms

empty mapped object type, intersection with optional object, K in never

🕗 Version & Regression Information

Tested on 4.0.5, 4.1.5, 4.2.0-beta, and the current nightly

⏯ Playground Link

Playground link

💻 Code

type AreAllStringsTs<T> = string extends T ? true : false;
type Test1 = AreAllStringsTs<{ foo?: number }>;                         // expected: false, actual: false
type Test2 = AreAllStringsTs<{ foo: number }>;                          // expected: false, actual: false
type Test3 = AreAllStringsTs<string | number>;                          // expected: true,  actual: true
type Test4 = AreAllStringsTs<{ [K in never]: any } & { foo?: number }>; // expected: false, actual: true  <-- FAIL
type Test5 = AreAllStringsTs<{ [K in never]: any } & { foo: number }>;  // expected: false, actual: false

🙁 Actual behavior

Test4 is true.

🙂 Expected behavior

Test4 is false.

Potential Workaround

If you have a mapped object type in which the type expression for the keys might reduce to never, add a conditional check to see if the keys set is empty. For example, I encountered this when working with a type that would make all keys of an object that could take on the value undefined optional:

type Optionalize<T> =
    { [K in RequiredKeys<T>]: T[K] }
    & { [K in NonRequiredKeys<T>]?: T[K] };

Adding a check for never made the behavior consistent with what I was expecting when every field of the input type accepted undefined:

type Optionalize<T> =
    (RequiredKeys<T> extends never ? {} : { [K in RequiredKeys<T>]: T[K] })
    & { [K in NonRequiredKeys<T>]?: T[K]; };

提供されたPlaygroundリンクから始めて、Test4とTest5の違いを再現してください。空のmapped typeがoptionalおよびrequiredのオブジェクト型と交差したときに、コンパイラーがどのように単純化するかを追跡してください。Test4がfalseと評価され、その他の報告された期待値が変更されない状態になれば完了です。