Union of functions with typed `this`, fails with no parameters

TypeScript Version: 4.0.2

Code

interface Context {
  hello: boolean
}

type OptionsTest =
      | ((this: Context) => void)
      | ((this: Context) => PromiseLike<any>)
      | ((this: Context, done: boolean) => void)

function container (test: OptionsTest) {

}

// This does not work!

container(function () {
  console.log(this.hello);
})

// This works!

container(function (done) {
  console.log(this.hello);
})

// This works!

type Single = (this: Context) => void;
function SingleContainer (test: Single) {}
SingleContainer(function () {
  console.log(this.hello)
})

Expected behavior:

Correctly provide this type to (typed union) functions without arguments.

Actual behavior:

No this type is being provided for functions without arguments.

Playground Link

StackOverflow Link

Search Terms

• contextually typed
• this parameter
• union of functions

Start with the linked Playground reproduction and compare contextual typing for the union in OptionsTest with the single-function Single type. Trace the compiler's handling of contextual typing, typed this parameters, and unions of functions. Done means the no-parameter callback receives Context.this while the existing parameterized and single-function cases continue to work, with regression coverage added where appropriate.