Fantomas duplicates commented line at the end of the file

[dbrattli]

Issue created from fantomas-online

Code

/// Returns `unit` if validation was successful otherwise will throw an `Exception`.

Result

/// Returns `unit` if validation was successful otherwise will throw an `Exception`.
/// Returns `unit` if validation was successful otherwise will throw an `Exception`.

Problem description

Fantomas seems to be duplicating commented lines if they are at the end of the file.

Extra information

• The formatted result breaks my code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas master branch at 2022-09-10T06:40:11Z - e1bd963

Default Fantomas configuration

_{Did you know that you can ignore files when formatting from fantomas-tool or the FAKE targets by using a .fantomasignore file?}

[nojaf]

This doesn't happen when you use // which you should as placing the /// at the end of a file already yields XML comment is not placed on a valid language element warning.

[github-actions]

🤖 This is an automated response from Repo Assist.

I've investigated this bug and found the root cause. Although a misplaced /// comment produces a compiler warning, Fantomas should still format it faithfully rather than duplicating it.

Root cause: In collectCodeComments (Parse.fs in the vendored FCS), when a /// comment appears at the end of a file, the lexer saves it in both CommentStore (via SaveSingleLineComment) and XmlDocStore (via trySaveXmlDoc → SaveXmlDocLine). When these two stores are combined, the same comment appears twice at the same source position.

This is confirmed reproducible in the current main branch.

Fix: Deduplicate the combined comment list by (StartLine, StartColumn) in collectCodeComments — no two comments can legitimately occupy the same source position.

I've opened a PR with the fix and a regression test: [PR link will appear here once created].

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