Using multiple extra accessors causes type errors arising from union types

Describe the bug 🐛

In TypeScript, if I attempt to use two extra accessors with different return types, the types for the computed values seem to be a union over the return types of all of the extra accessors, rather than just the return type of the accessor in use.

(Related: those types also include undefined, even in cases where the value is required, which seems wrong — but that’s a separate issue I guess.)

To Reproduce 📝

Here we use the asEmail example given in README.md, and we add another extra accessor called asZero which always returns zero. So the type for asEmail is string, and the type for asZero is number:

process.env.ADMIN = '[e-mail]'
 
const asEmail: ExtensionFn<string> = (value) => {
  const split = String(value).split('@')
  if (split.length !== 2) {
    throw new Error('must contain exactly one "@"')
  }
  return value
}

const asZero: ExtensionFn<number> = (value) => 0;

const env = from(process.env, {asEmail, asZero})

Then the type required for a value using either of these extra accessors is number | string | undefined, so I have to write the following:

const validEmail: number | string | undefined = env.get('ADMIN').asEmail()
const zero: number | string | undefined = env.get('ADMIN').asZero()

Expected behaviour 🤷‍♂️🤷

I would expect this to work / not fail type checking:

const validEmail: string | undefined = env.get('ADMIN').asEmail()
const zero: number | undefined = env.get('ADMIN').asZero()

(In fact, as mentioned above, I’d love to not have to include undefined here; AFAICS asEmail always either returns a string or raises an error, so why not…

const validEmail: string = env.get('ADMIN').asEmail()
const zero: number = env.get('ADMIN').asZero()

… but again, that’s secondary to the issue I really want to raise, which is the union thing.)

Environment (please complete the following information) 💻:

• OS: macOS Mojave 10.14.6
• Runtime: Chrome 79.0.3945.130
• Version 6.0.4

[evanshortiss]

@gimbo I’ve fixed the undefined now also, thank you!

[gimbo]

@evanshortiss Hi Evan - didn’t get chance to look at this over the weekend but I’ve just just checked and it does indeed seem to be fixed for my use case. The undefineds are still there but the number/string clash is gone. Thanks!