highlightjs/highlight.js

C++ : Trailing return types in function declaration result in illegal language match

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#3,673 opened on Dec 8, 2022

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Description

Describe the issue/behavior that seems buggy

hljs.highlightAuto does not work on the following example, while hljs.highlightElement does.

Sample Code or Instructions to Reproduce

hljs.highlightAuto("auto main() -> int {\n    auto i = 42; // test\n    return i;\n}", [ 'cpp' ]) 

// result :
{
  "language": "cpp",
  "value": "auto main() -> int {\n    auto i = 42; // test\n    return i;\n}",
  "illegal": true,
  "relevance": 0,
  "_illegalBy": {
    "message": "Illegal lexeme \"-\" for mode \"function\"",
    "index": 11,
    "context": "auto main() -> int {\n    auto i = 42; // test\n    return i;\n}",
    "mode": {},
    "resultSoFar": ""
  },
 ...
}

This issue seems to be related to the arrow -> (feature name : trailing return type), which is legal in post-modern C++.

See the documentation of function declaration, section (2) (C++11), here on cppreference :

Expected behavior

  • Consistency between highlightAuto and highlightElement behaviors
  • Proper modern and post-modern C++ synthaxes match & detection

Contributor guide

Tech stack
cppjavascript
Domain
frontend
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 familiarityC++ syntaxhighlight.js internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Investigate the C++ language definition file (src/languages/cpp.js). Focus on the 'function' mode where the illegal ' ' character is matching incorrectly. Understand how trailing return types are parsed and adjust the regex or mode rules to recognize the arrow ' >' as part of a function declaration rather than an illegal token. Look at the highlightAuto detection logic to ensure consistency with highlightElement.