Narrowing a union from generic fails when union is not the first generic

[luke-john]

TypeScript Version:

• 2.9.0-dev.20180330
• 2.8.1
• 2.7.2

Search Terms:

• generic order
• map index generic

Code

type Tags  = { foo: ''; bar: ''; baz: ''; }
type Tag = keyof Tags 

type GetTag<UsedTag extends keyof Tags> = keyof { [tag in UsedTag]: any } 

type DoThingVersionOne =
  <UsedTag extends Tag = Tag, Props = {}>(tag: UsedTag) => GetTag<UsedTag>

const doThingVersionOne: DoThingVersionOne = {} as any

const fooOne = doThingVersionOne('foo')                       // expected | type = "foo"
const fooOneITag = doThingVersionOne<'foo'>('foo')            // expected | type = "foo"
const fooBarITagIProps = doThingVersionOne<'foo', {}>('foo')  // expected | type = "foo"

type DoThingVersionTwo =
    <Props = {}, UsedTag extends Tag = Tag>(tag: UsedTag) => GetTag<UsedTag>

const doThingVersionTwo: DoThingVersionTwo = {} as any

const fooTwo = doThingVersionTwo('foo')                       // expected | type = "foo"
const fooTwoITag = doThingVersionTwo<{}>('foo')               // unexpected | type "foo" | "bar" | "baz" 
const fooTwoITagIProps = doThingVersionTwo<{}, 'foo'>('foo')  // expected | type "foo" 

Expected behavior:

The order of a generic does not effect it's behaviour when narrowing from a union.

Actual behavior:

The order of a generic does effect it's behaviour when narrowing from a union.

As described below when no generics are passed typescript infers all of them, when any number of generics are passed typescript partially instantiates all of them using the defaults provided.

Playground Link:

Playground Reproduction

Related Issues:
#17713
#21631
#12424

[jack-williams]

The order matters because of the default values you have and in the unexpected case you're partially instantiating the generics. The following two are equivalent and give the same result.

const fooTwoITagWithDefault = doThingVersionTwo<{}>('foo') //  "foo" | "bar" | "baz" 
const fooTwoITag = doThingVersionTwo<{}, Tag>('foo') //  "foo" | "bar" | "baz" 

I believe partial application of generics turns off inference for those that are omitted (they fall back to their default value). I guess the issue you're saying is that it should try and infer missing default type parameters?

[luke-john]

Ahh, right. I was thinking when providing generics, only the ones passed would be instantiated.

Happy to close this, thanks for your quick response and explanation.