google/closure-compiler

Parser error on "combining character" (U+0307)

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#3,639 opened on Jul 14, 2020

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Description

There seems to be an issue with parsing composite character sequences.

Composite character İ consist of the base character and combining character sequence (U+0049)(U+0307)
Using this character causing the compiler to fail with the parser error.

Example code

var bar = {
İ: "foo"
};

Compiler error

input0:2: ERROR - [JSC_PARSE_ERROR] Parse error. Character '̇' (U+0307) is not a valid identifier start char
  2| İ: "foo"
      ^

1 error(s), 0 warning(s)

Demo

Example

Contributor guide

Tech stack
javascript
Domain
tooling
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
JavaScript syntaxUnicode combining characters
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The error occurs in the JavaScript lexer/parser when validating identifier start characters. Investigate the lexer implementation to understand why combining character U+0307 is rejected. Look at the Unicode character validation logic, likely in the scanner or parser source files (e.g., JSTokenizer.java). The fix may involve allowing combining characters that follow a valid base character.