Object is possibly 'undefined' when accessing Partial object via bracket notation

[nivzelber]

I can't understand why i can gain access to a Partial object this way a.b && a.b.c
but not this way a[keyof A] && a[keyof A].b (when typeof a is A).

I have a use-case in which I can't use optional chaining. I want to conditionally render a React element like so {myObj[key] && <p>myObj[key].message</p>} when the type variable key is of type keyof myObj. See example below to better understand.

TypeScript Version: 3.8.3

Search Terms:

• cant access partial type with Bracket notation
• Bracket notation on Partial
• access to partial type with Bracket notation failes to evaluate

Code

export interface MyType {
  a: number;
  b: string;
  c: Date;
}

interface MyOtherType {
  message: string;
}

const myObj: Partial<Record<keyof MyType, MyOtherType>> = {
  a: { message: "my message" },
  b: { message: "my message" },
  c: { message: "my message" }
};

myObj.a.message // not allowed - `Object is possibly 'undefined'`
myObj.a?.message // allowed
myObj.a && myObj.a.message // allowed

const myKey: keyof MyType = "a";
myObj[myKey].message // not allowed - `Object is possibly 'undefined'`
myObj[myKey]?.message // allowed
myObj[myKey] && myObj[myKey].message // not allowed - `Object is possibly 'undefined'` - why?

Expected behavior:
last line should work
Actual behavior:
Object is possibly 'undefined' error thrown when running tsc

Playground Link: https://codesandbox.io/s/mystifying-framework-5110c

Related Issues: #29642 is very similar but was closed for a being a duplicate of an issue from 2016 (?!). I don't think they are even related.

[MartinJohns]

Duplicate #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.