Type inference does not work for enums and calculated props

[pm-nsimic]

TypeScript Version: 3.1.6

Calculated props, Spread operator, Enum keyed types

Code

enum Test {
  param = 'X',
  other = 'Y',
  last = 'Z',
};

type TSomeOtherType = {
  readonly someProp: number;
  readonly orOther: string;
};

type TKeyedByEnum = {
  [key in Test]: TSomeOtherType | null;
};

// const throwsCompilerError: TKeyedByEnum = {
//   [Test.param]: 123,
// };

Object.keys(Test).reduce(
    (acc, key: Test): TKeyedByEnum => {
      const doesntThrow: TKeyedByEnum = {
        ...acc,
        [key]: 123,
      };
      return doesntThrow;
    },
    {} as TKeyedByEnum
);

Expected behavior:

Typescript compiler throws an error on the object doesntThrow.

Actual behavior:

There is no typescript error on the object doesntThrow. The resulting object of type TKeyedByEnum contains values of type number.

Playground Link:
http://www.typescriptlang.org/play/#src=enum%20Test%20%7B%0D%0A%20%20param%20%3D%20'X'%2C%0D%0A%20%20other%20%3D%20'Y'%2C%0D%0A%20%20last%20%3D%20'Z'%2C%0D%0A%7D%3B%0D%0A%0D%0Atype%20TSomeOtherType%20%3D%20%7B%0D%0A%20%20readonly%20someProp%3A%20number%3B%0D%0A%20%20readonly%20orOther%3A%20string%3B%0D%0A%7D%3B%0D%0A%0D%0Atype%20TKeyedByEnum%20%3D%20%7B%0D%0A%20%20%5Bkey%20in%20Test%5D%3A%20TSomeOtherType%20%7C%20null%3B%0D%0A%7D%3B%0D%0A%0D%0Aconst%20throwsCompilerError%3A%20TKeyedByEnum%20%3D%20%7B%0D%0A%20%20%5BTest.param%5D%3A%20123%2C%0D%0A%7D%3B%0D%0A%0D%0Aconst%20t%20%3D%20Object.keys(Test).reduce(%0D%0A%20%20%20%20(acc%2C%20key%3A%20Test)%3A%20TKeyedByEnum%20%3D%3E%20%7B%0D%0A%20%20%20%20%20%20const%20doesntThrowCompilerError%3A%20TKeyedByEnum%20%3D%20%7B%0D%0A%20%20%20%20%20%20%20%20...acc%2C%0D%0A%20%20%20%20%20%20%20%20%5Bkey%5D%3A%20123%2C%0D%0A%20%20%20%20%20%20%7D%3B%0D%0A%20%20%20%20%20%20return%20doesntThrowCompilerError%3B%0D%0A%20%20%20%20%7D%2C%0D%0A%20%20%20%20%7B%7D%20as%20TKeyedByEnum%0D%0A)%3B%0D%0A%0D%0Aconsole.log(t)%3B

Related Issues:
#27704

[weswigham]

Ref #26797 which if it doesn't already fix this, should be able to fix this with only small modifications.

[kwasimensah]

Just ran into a version of this https://www.typescriptlang.org/play/index.html#code/MYewdgzgLgBApmArgWxgMRCGBvAUDAmAQRgF4YAiIigGlwF9ddRJYo5oAuGAJTlABOAEwA8GEDRhJkAIzgCAfGRyVq3AIwx6AbiYsIIADZwAdIZABzABTtoAbXEmiAXQCUu3ADMQAmFf2wANZwAJ4wAJZgMLZQrjj4hPpGpubWMXbBIW669EA