Can't affect unknown to class with private field: Cannot read property 'flags' of undefined

[EmrysMyrddin]

TypeScript Version: 3.8.2

Search Terms: 'flags' of undefined

Code

export class Class {
  #field: any
}

const task: Class = {} as unknown

Expected behavior:

To give the compile error: Type 'unknown' is not assignable to type 'Class'.

Actual behavior:

Crashes with :

/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:80993
                throw e;
                ^

TypeError: Cannot read property 'flags' of undefined
    at reportUnmatchedProperty (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:40618:38)
    at propertiesRelatedTo (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:40663:25)
    at structuredTypeRelatedTo (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:40349:34)
    at recursiveTypeRelatedTo (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:40096:53)
    at isRelatedTo (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:39777:38)
    at checkTypeRelatedTo (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:39487:26)
    at checkTypeRelatedToAndOptionallyElaborate (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:38806:24)
    at checkTypeAssignableToAndOptionallyElaborate (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:38800:20)
    at checkVariableLikeDeclaration (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:52072:25)
    at checkVariableDeclaration (/Users/valentin/Zenika/Projects/cuillere/node_modules/typescript/lib/tsc.js:52136:20)

Playground Link: I didn't found a way to reproduce this behaviour in the playground.

Related Issues: #36871 and #36677