Possible regression in typechecker for Required type

[Smrtnyk]

Bug Report

Since upgrade to 4.6 from 4.4 I have an error with Required type
full repro here
https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgGIHt3IN4ChkHIBGcUA-AFzIDOYUoA5rgL664wCuICYw6IyGJgA8AFWQQAHpBAATasgDWEAJ7oYaTAD4AFAAco6PVVEAaQVQBKEAI4dgUCLOEZ0WgJQ58hElB0wAbQMjAF13AG4WNk5uXn5iUh0wKlp6EAZPPEJkBH5qdAAbCAA6AvQGJIiWIA

The error was not happening on 4.4 and seems like also not in 4.5.
I didn't really see something obvious in release notes on what would cause this change.

🔎 Search Terms

Went through issues in ts repo to see if there was something already open

🕗 Version & Regression Information

4.6

Please keep and fill in the line that best applies:
-->

• This is a crash
• This changed between versions 4.4__ and 4.6___

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo {
    bar?: string
}

function foo<T extends keyof Foo>(prop: T, f: Required<Foo>) {
    bar(f[prop]);
}

function bar(t: string) {
    console.log(t);
}

🙁 Actual behavior

It complains that f[prop] can be undefined, but it is wrapped in Required which should have all keys from Foo as non optional

🙂 Expected behavior

Well, I guess it should not complain about undefined, if I properly understand the meaning of the Required generic

[DerGernTod]

this only happens with strictNullChecks enabled, in case this is of any help

[RyanCavanaugh]

Bisected to #47953

[ahejlsberg]

This and #48246 are caused by the same core issue. We need to exclude mapped types with optionality modifiers and/or as clauses from the new constraint logic introduced in #47370.