Hover hint is invalid if there are multiple type aliases with same parameters

[zaki-yama]

TS Template added by Matt Bierner (@mjbvz)

TypeScript Version: 3.9.3

Search Terms

• quick info
• alias / aliases

---

• VSCode Version: 1.45.1
• OS Version: Darwin x64 18.7.0

Steps to Reproduce:

1. Write the following TypeScript code

type Foo = string | number;
type Bar = string | number;

function someFunc(params: {
  foo: Foo,
  bar: Bar,
}) {
  //
}

// then try to call the function `someFunc`
someFunc(

Here I defined two type aliases, Foo and Bar.
Both have the same parameters, string | number.

2. Hover over the someFunc and show popup
3. The expected type of parameter bar is Bar, but actually it was Foo

It's also happened in Signature help.

Does this issue occur when all extensions are disabled?: Yes

[IllusionMH]

Looks like duplicate of #36761

/assign mjbvz

[mjbvz]

Also related to #34778

The issue is that Foo and Bar are structurally equivalent from the type system's point of view

[DanielRosenwasser]

Yes, when we try to display the type

{
  foo: Foo,
  bar: Bar,
}

we are actually working with the type represented by

{
  foo: string | number,
  bar: string | number,
}

When we see string | number, we say "oh, yeah, I saw an alias for that once" and use the first we encountered: Foo.

[zaki-yama]

For your information, when I tried to open the same file in IntelliJ IDEA, hover hint showed the type alias correctly.