[ts 4.7] Support `extends` constraints for `infer` by sosukesuzuki · Pull Request #14476 · babel/babel

sosukesuzuki · 2022-04-20T05:17:27Z

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 · 2022-04-20T05:20:58Z

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

nicolo-ribaudo · 2022-04-20T13:26:52Z

packages/babel-parser/src/plugins/typescript/index.js

      this.expectContextual(tt._infer);
      const typeParameter = this.startNode();
      typeParameter.name = this.tsParseTypeParameterName();
+      typeParameter.constraint = this.tsEatThenParseType(tt._extends);


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?

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 · 2022-04-20T13:27:27Z

packages/babel-parser/test/fixtures/typescript/types/infer-with-constraints/options.json

@@ -0,0 +1,3 @@
+{
+  "BABEL_8_BREAKING": false


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

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

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 · 2022-04-22T06:05:34Z

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

Do you have a better solution?

JLHwung · 2022-04-22T19:03:13Z

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 · 2022-04-24T14:03:53Z

@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 · 2022-05-03T09:12:48Z

packages/babel-parser/test/fixtures/typescript/types/infer-with-constraints-babel-7/output.json

+                  "type": "TSTypeParameter",
+                  "start":30,"end":46,"loc":{"start":{"line":1,"column":30,"index":30},"end":{"line":1,"column":46,"index":46}},
+                  "name": "U",
+                  "constraint": {


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.

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)

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 · 2022-05-03T09:13:56Z

packages/babel-parser/test/fixtures/typescript/types/infer-with-constraints/input.ts

@@ -0,0 +1,5 @@
+type X3<T> = T extends [infer U extends number] ? MustBeNumber<U> : never;


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;

sosukesuzuki added PR: New Feature 🚀 A type of pull request used for our changelog categories pkg: parser area: typescript labels Apr 20, 2022

existentialism approved these changes Apr 20, 2022

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nicolo-ribaudo reviewed Apr 20, 2022

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nicolo-ribaudo added this to the v7.18.0 milestone Apr 20, 2022

JLHwung reviewed Apr 20, 2022

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sosukesuzuki force-pushed the infer-extends branch from f47bd4c to c9a8e60 Compare April 24, 2022 12:56

existentialism approved these changes Apr 26, 2022

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sosukesuzuki force-pushed the infer-extends branch from c9a8e60 to 1509f4e Compare May 1, 2022 07:12

sosukesuzuki mentioned this pull request May 1, 2022

[Bug]: @babel/parser doesn't throw a recoverable error for infer that is outside of conditional types #14511

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nicolo-ribaudo reviewed May 3, 2022

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sosukesuzuki force-pushed the infer-extends branch from 1509f4e to a965c3e Compare May 5, 2022 06:21

nicolo-ribaudo approved these changes May 6, 2022

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sosukesuzuki mentioned this pull request May 6, 2022

Support TypeScript 4.7 prettier/prettier#12640

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sosukesuzuki added 9 commits May 17, 2022 23:44

Parse constraint for infer

d1bf2db

Add tests

e808887

Set "BABEL_8_BREAKING": false for test

9355fd3

Add tests for babel 8

c3430d0

Fix type of TsInferType.typeParameter

f26ffe5

Add tests for babel-generator

9431df4

Support infer extends and conditional types

f6168aa

Update snapshots for BABEL_8

e552289

Refactoring accoring to tsc implementation

bfecf94

Add more tests

7c29839

nicolo-ribaudo force-pushed the infer-extends branch from 828d3ad to 7c29839 Compare May 17, 2022 21:45

nicolo-ribaudo changed the title ~~feat(babel-parser): support TS 4.7 extends constraints for infer~~ [ts 4.7] Support extends constraints for infer May 17, 2022

nicolo-ribaudo merged commit 59d24a4 into babel:main May 17, 2022

github-actions bot added the outdated A closed issue/PR that is archived due to age. Recommended to make a new issue label Aug 17, 2022

github-actions bot locked as resolved and limited conversation to collaborators Aug 17, 2022

		@@ -0,0 +1,5 @@
		type X3<T> = T extends [infer U extends number] ? MustBeNumber<U> : never;

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sosukesuzuki commented Apr 20, 2022 •

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