{} 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]; };

Start with the provided Playground link and reproduce the difference between Test4 and Test5. Trace how the compiler simplifies the empty mapped type when it is intersected with optional and required object types. Done means Test4 evaluates to false while the other reported expectations remain unchanged.