Can't assign string to string | null for object in immutable map

[Methuselah96]

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Migrated from don't!
Originally created by @JoshuaLamusga on Thu, 10 Sep 2020 20:24:13 GMT

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What happened

With typescript 3.9.7 and immutable-js version 3.8.2, the following code unexpectedly throws an error:

type TestAbcType = IImmutableMap<{ a: boolean, b: string | null }>
const testAbc = createImmutableMap({ a: true, b: null })
const testXyz: TestAbcType = testAbc

With the error being:

Type 'IImmutableMap<{ a: boolean; b: null; }>' is not assignable to type 'TestAbcType'.
  Type 'string | null' is not assignable to type 'null'.
    Type 'string' is not assignable to type 'null'.

This is surprising because according to the type definition, b can be a string or null. In this case, b is null and should satisfy the condition of string | null. This code works if it's not wrapped with IImmutableMap, indicating the problem lies with this library.

[Methuselah96]

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Migrated from don't reference!
Original comment by @conartist6 on Wed, 16 Sep 2020 13:25:28 GMT

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You hav not included the definition of IImmutableMap.