[ts 4.7] Support extends constraints for infer

[sosukesuzuki]

Q	A
Fixed Issues?	N/A
Minor: New Feature?	Y
Tests Added + Pass?	Y
License	MIT

Support https://devblogs.microsoft.com/typescript/announcing-typescript-4-7-beta/#extends-constraints-on-infer-type-variables

type FirstString<T> =
    T extends [infer S extends string, ...unknown[]]
        ? S
        : never;

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/51953/

[nicolo-ribaudo]

Can you also add this to the type definitions in https://github.com/babel/babel/blob/main/packages/babel-parser/src/types.js, in @babel/types and in @babel/generator?

[sosukesuzuki]

• I've fixed type definition in @babel/parser a837b29
• I've added tests in @babel/generator. bfce230
• The type for TSInferType that is defined in @babel/types supports constraint field already. So I think we don't need to change @babel/types in this PR.

[nicolo-ribaudo]

Is this behavior different in Babel 8? If it is, can you also add a Babel 8 test?

[sosukesuzuki]

In babel 8, the AST for type parameters will be changed. I'll add tests for babel 8 later!

[sosukesuzuki]

7c7d753 👍

[JLHwung]

I think we need to handle the precedence of extends vs ?. Can you add a new test case

type X<U, T> = T extends [infer U extends T ? U : T] ? U : T;

It is valid in tsc 4.7.0.

[sosukesuzuki]

@JLHwung I've tried to support precedence of infer extends and conditional types ee9c29f.

Do you have a better solution?

[JLHwung]

Can you add a new test case?

type X<U, T> = [(infer U extends T)?];

is invalid according to tsc. Currently Babel 7 was passing while Babel 8 is throwing.

In microsoft/TypeScript#48112, tsc introduced a new parsing context DisallowConditionalTypesContext and apply it to extends type. Since TS doesn't have language spec anyway, I think we can follow tsc's approach and add a new production parameter accordingly.

[sosukesuzuki]

@JLHwung Thank you for your review. I've refactored according to tsc implementation. c9a8e60

is invalid according to tsc.

It is semantically invalid, but seems to parse successfully(TS Playground link). Should Babel throw a recoverable error for it? If Babel needs to throw a recoverable error for it, I think the fix should be in a separate PR. This is because even with infer without extends, tsc throws a similar semantic error(TS Playground link), and Babel does not throw an error as well(Babel repl link).

[nicolo-ribaudo]

I am really surprised by this placement. I'd expect constraint to be on TSInferType, not on TSTypeParameter. The new syntax is infer T extends U, not just T extends U.

[sosukesuzuki]

I think instead of adding a new constraint field to TSInferType, we should add a constraint to the typeParamter of TSInferType. Because it is the type parameter, not the infer, that is constrained.

FYI, the AST in TypeScript Compiler adds constaint to TypeParameter. (TypeScript AST Viewer Link)

[nicolo-ribaudo]

Tbh, I really dislike many choices that the TS team made with their AST 😛

However, I realized that your AST design follows the shape we already have for type X<A extends B> so it's fine.

[nicolo-ribaudo]

Can you also add some tests without tuples?

type X<U, T> = T extends infer U extends number ? U : T;
type X<U, T> = T extends (infer U extends number ? U : T) ? U : T;