[3.5.1] Type 'NonNullable<Partial<Config>[T]>' is not assignable to type 'Config[T]'

TypeScript Version: 3.5.1, 3.6.0-dev.20190530

Search Terms:

partial nonnullable generic mapped strictNullChecks

Code

interface Config {
  string: string
  // There need to be different types in here for the error to occur
  number: number
}

function getConfigOrDefault<T extends keyof Config>(
  userConfig: Partial<Config>,
  key: T,
  defaultValue: Config[T]
): Config[T] {
  const userValue = userConfig[key] // Partial<Config>[T]
  const explicitAssertion = userConfig[key] as Config[T] | undefined // allowed

  // Partial<Config>[T] - already incorrect in 3.4.5?
  const simpleCheck = userValue ? userValue : defaultValue

  // 3.4.5: Config[T]
  // 3.5.1, 3.6.0-dev.20190530: Config[T] | NonNullable<Partial<Config>[T]>
  const assertedCheck = userValue ? userValue! : defaultValue

  return assertedCheck
}

Expected behavior:

No error

Actual behavior:

Type 'Config[T] | NonNullable<Partial<Config>[T]>' is not assignable to type 'Config[T]'.
  Type 'NonNullable<Partial<Config>[T]>' is not assignable to type 'Config[T]'.
    Type 'Partial<Config>[T]' is not assignable to type 'Config[T]'.
      Type 'Partial<Config>' is not assignable to type 'Config'.
        Types of property 'string' are incompatible.
          Type 'string | undefined' is not assignable to type 'string'.
            Type 'undefined' is not assignable to type 'string'.
              Type 'Partial<Config>[T]' is not assignable to type 'string & number'.
                Type 'Partial<Config>[T]' is not assignable to type 'string'.
                  Type 'string | number' is not assignable to type 'Config[T]'.
                    Type 'string' is not assignable to type 'Config[T]'.
                      Type 'string' is not assignable to type 'string & number'.
                        Type 'string' is not assignable to type 'number'.
                          Type 'NonNullable<Partial<Config>[T]>' is not assignable to type 'string & number'.
                            Type 'Partial<Config>[T]' is not assignable to type 'string & number'.
                              Type 'string | number | undefined' is not assignable to type 'string & number'.
                                Type 'undefined' is not assignable to type 'string & number'.
                                  Type 'undefined' is not assignable to type 'string'.
                                    Type 'Partial<Config>[T]' is not assignable to type 'string'.
                                      Type 'string | number' is not assignable to type 'string & number'.
                                        Type 'string' is not assignable to type 'string & number'.
                                          Type 'string' is not assignable to type 'number'.
                                            Type 'NonNullable<Partial<Config>[T]>' is not assignable to type 'string'.
                                              Type 'Partial<Config>[T]' is not assignable to type 'string'.
                                                Type 'string | number | undefined' is not assignable to type 'string'.
                                                  Type 'undefined' is not assignable to type 'string'.
                                                    Type 'string | number' is not assignable to type 'string'.
                                                      Type 'number' is not assignable to type 'string'.ts(2322)

with strictNullChecks: true

Playground Link:

https://www.typescriptlang.org/play/index.html#src=interface%20Config%20%7B%0D%0A%20%20string%3A%20string%0D%0A%20%20%2F%2F%20There%20need%20to%20be%20different%20types%20in%20here%20for%20the%20error%20to%20occur%0D%0A%20%20number%3A%20number%0D%0A%7D%0D%0A%0D%0Afunction%20getConfigOrDefault%3CT%20extends%20keyof%20Config%3E(%0D%0A%20%20userConfig%3A%20Partial%3CConfig%3E%2C%0D%0A%20%20key%3A%20T%2C%0D%0A%20%20defaultValue%3A%20Config%5BT%5D%0D%0A)%3A%20Config%5BT%5D%20%7B%0D%0A%20%20const%20userValue%20%3D%20userConfig%5Bkey%5D%20%2F%2F%20Partial%3CConfig%3E%5BT%5D%0D%0A%20%20const%20explicitAssertion%20%3D%20userConfig%5Bkey%5D%20as%20Config%5BT%5D%20%7C%20undefined%0D%0A%0D%0A%20%20%2F%2F%20Partial%3CConfig%3E%5BT%5D%20-%20already%20incorrect%20in%203.4.5%3F%0D%0A%20%20const%20simpleCheck%20%3D%20userValue%20%3F%20userValue%20%3A%20defaultValue%0D%0A%0D%0A%20%20%2F%2F%203.4.5%3A%20Config%5BT%5D%0D%0A%20%20%2F%2F%203.5.1%2C%203.6.0-dev.20190530%3A%20Config%5BT%5D%20%7C%20NonNullable%3CPartial%3CConfig%3E%5BT%5D%3E%0D%0A%20%20const%20assertedCheck%20%3D%20userValue%20%3F%20userValue!%20%3A%20defaultValue%0D%0A%0D%0A%20%20return%20assertedCheck%0D%0A%7D%0D%0A

(error does not occur there - which TS version does the playground run on?)

Related Issues:

These issues looked similar but were already reported for earlier versions of TypeScript:

#27456
#30706

My current workaround for this issue:

interface Config {
  string: string
  // There need to be different types in here for the error to occur
  number: number
}

// This compiles just fine in 3.7.2
function getValueOrDefault<K extends keyof T, T>(obj: Partial<T>, key: K, defaultValue: T[K]): T[K] {
    return obj[key] ?? defaultValue;
}

function getConfigOrDefault<T extends keyof Config>(
  userConfig: Partial<Config>,
  key: T,
  defaultValue: Config[T]
): Config[T] {
  // return userConfig[key] ?? defaultValue; // I would expect this to work, as stated above
  return getValueOrDefault(userConfig, key, defaultValue);
}

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[3.5.1] Type 'NonNullable<Partial<Config>[T]>' is not assignable to type 'Config[T]' #31675

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[3.5.1] Type 'NonNullable<Partial<Config>[T]>' is not assignable to type 'Config[T]' #31675

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wezleytsai commented on Nov 11, 2019

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