Microsoft/TypeScript

type.isLiteral() returns false for boolean literals

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#26,075 opened on Jul 31, 2018

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 (7 comments) (2 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: APIDomain: Literal TypesHelp Wanted

Description

TypeScript Version: 3.1.0-dev.20180728 (new in 3.0.1)

Search Terms: isLiteral()

Code

  1. Create a boolean literal in the compiler api.
  2. Call isLiteral() on it.

Change seems to be done here:

https://github.com/Microsoft/TypeScript/commit/e46d214fceb72cc76145779543bf871ce9ff66b0#diff-233e1126c0abc811c4098757f9e4516eR428

Expected behavior: true for a boolean literal type (true or false)

Actual behavior: false

I have a few tests for assumptions about the compiler api in ts-simple-ast and this one started failing. Maybe it's not a bug, but I thought I would log it anyway to find out why this behaviour changed.

Is it because it doesn't have the Unit flag while string and number literals do? For what it's worth, in checker.ts it will internally return true for boolean literals in isLiteralType.

Contributor guide

Tech stack
typescript
Domain
developer experiencetooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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 compiler API basicsunderstanding of literal types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Look at the commit e46d214fceb72cc76145779543bf871ce9ff66b0, specifically the change in checker.ts around line 428 where isLiteralType is defined. The issue is that boolean literals (true/false) are not recognized as literals because they lack the Unit flag. Investigate if adding the Unit flag for boolean literals or adjusting the condition in isLiteralType to include boolean literals would fix the behavior. Check the linked commit and read the current implementation of isLiteralType in the checker.ts file.