Microsoft/TypeScript

Generic type sometimes returns never instead of actual template parameter type

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#41,778 创建于 2020年12月2日

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 (6 评论) (5 反应) (0 负责人)TypeScript (48,455 star) (6,726 fork)batch import
BugDomain: Conditional TypesExperimentation NeededHelp Wanted

描述

TypeScript Version: 3.6+

Search Terms: enum, extends, never, template parameters

Code

enum FL4 { Absurd, False, True, Unknown }

type Classify< Left, Right > = Left extends Right

	? Right extends Left
		? [ Left , '==', Right ]
		: [ Left , '<:', Right ]
	
	: Right extends Left
		? [ Left, ':>', Right ]
		: [ Left , '!=', Right ]

Actual at 3.6+

type Absurd_is_never_wtf = Assert<
    Classify< FL4.Absurd, 0 >,
    [ never, '<:', 0 ]
>

type One_is_never_wtf = Assert<
    Classify< FL4.Absurd, 1 >,
    [ FL4.Absurd, ':>', never ]
>

Expected and before 3.6

type Absurd_is_Absurd = Assert<
    Classify< FL4.Absurd, 0 >,
    [ FL4.Absurd, '==', 0 ]
>

type One_is_One = Assert<
    Classify< FL4.Absurd, 1 >,
    [ FL4.Absurd, ':>', 1 ]
>
```

贡献者指南

技术栈
typescript
领域
backend
议题类型
bug
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
5
预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。
over 1 week
活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。
stale
清晰度议题是否清楚说明期望改动、验收标准和下一步。
clear
前置要求
TypeScript generics and conditional typesTypeScript compiler codebase
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
15
研究方向
The issue is a regression in TypeScript's type inference for conditional types between versions 3.5 and 3.6. The bug causes generic types to incorrectly resolve to 'never' when used with enum members. To fix, investigate changes in the TypeScript compiler's type checker (checker.ts) related to distributive conditional types and assignability. Look for commits in the TypeScript repository between those versions that modified handling of conditional types. Review existing comments or linked PRs for additional context.
Generic type sometimes returns never instead of actual template parameter type · Microsoft/TypeScript#41778 | Good First Issue