Microsoft/TypeScript

Generic type sometimes returns never instead of actual template parameter type

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#41,778 opened on Dec 2, 2020

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 (6 comments) (5 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: Conditional TypesExperimentation NeededHelp Wanted

Description

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 ]
>
```

Contributor guide

Tech stack
typescript
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript generics and conditional typesTypeScript compiler codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
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.