Promise's `then` method allows to get wrong type for term depends of its type-argument.

[Renegatto]

Bug Report

🔎 Search Terms

then, Promise, async

🕗 Version & Regression Information

v 4.2.3
v 4.3.0-dev.20210412

⏯ Playground Link

Playground link with relevant code

💻 Code

const foo: Promise<Promise<number>> = 
    Promise.resolve<Promise<number>>(Promise.resolve(666))
//                  ^^^^^^^^^^^^^^^ type argument
foo.then((x: Promise<number>) => console.log(typeof x))
//        ^^^^^^^^^^^^^^^^^ thats actually wrong type for 'x', 'x' is actually a 'number'

🙁 Actual behavior

Output: "number"
This is wrong because x annotated as Promise<number> and it typechecks.

🙂 Expected behavior

Compilation error or output: "object"

[MartinJohns]

Duplicate of #27711. Promise<Promise<T>> should result in an error.

Your initial foo type is already wrong. It should be Promise<number>.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.