Removing optional modifier also removes `undefined` from value type

TypeScript Version: 3.3.3333

Search Terms: NonPartial, remove optional modifier

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

interface OptClass {
  opt?: number;
}

type NonPartialIsh = {[K in keyof OptClass]-?: OptClass[K] | undefined};

const test = {opt: undefined};

verify<NonPartialIsh>(test);  // should NOT be error, but shows error

function verify<T>(a: T) {}

Expected behavior:

• Should not have any error.
• NonPartialish.opt type should support undefined.

Actual behavior:

• Throws the following error:

ERROR(12,23): : Argument of type '{ opt: undefined; }' is not assignable to parameter of type 'NonPartialIsh'.
  Types of property 'opt' are incompatible.
    Type 'undefined' is not assignable to type 'number'.
Transpiled code follows despite errors.

• NonPartialish.opt type does not support undefined.

Playground Link: https://www.typescriptlang.org/play/#src=%2F%2F%20A%20*self-contained*%20demonstration%20of%20the%20problem%20follows...%0D%0A%2F%2F%20Test%20this%20by%20running%20%60tsc%60%20on%20the%20command-line%2C%20rather%20than%20through%20another%20build%20tool%20such%20as%20Gulp%2C%20Webpack%2C%20etc.%0D%0A%0D%0Ainterface%20OptClass%20%7B%0D%0A%20%20opt%3F%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Atype%20NonPartialIsh%20%3D%20%7B%5BK%20in%20keyof%20OptClass%5D-%3F%3A%20OptClass%5BK%5D%20%7C%20undefined%7D%3B%0D%0A%0D%0Aconst%20test%20%3D%20%7Bopt%3A%20undefined%7D%3B%0D%0A%0D%0Averify%3CNonPartialIsh%3E(test)%3B%20%20%2F%2F%20should%20NOT%20be%20error%2C%20but%20shows%20error%0D%0A%0D%0Afunction%20verify%3CT%3E(a%3A%20T)%20%7B%20%7D%0D%0A

Related Issues:

Start by running the self-contained example with TypeScript 3.3.3333 and compare the mapped type's behavior when removing the optional modifier. Trace how the compiler handles the property type in this mapped-type case; done means the example accepts {opt: undefined} and the resulting property type supports undefined.