Comment on last match case has wrong indentation

[stefan-schweiger]

Issue created from fantomas-online

Code

let test x =
    match x with
    | Value1 -> 1
    // | Value2 -> 2
    | Value3 -> 3

let test2 x =
    match x with
    | Value1 -> 1
    | Value2 -> 2
    // | Value3 -> 3

Result

let test x =
    match x with
    | Value1 -> 1
    // | Value2 -> 2
    | Value3 -> 3

let test2 x =
    match x with
    | Value1 -> 1
    | Value2 -> 2
// | Value3 -> 3

Problem description

If you comment a match case the alignment fits with the rest of the cases, but if you comment the last match case the alignment will be at the start of the line. (Happens with both main and dallas)

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 main branch at 2022-12-03T22:59:05Z - 47c6ac6

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]

Hi there, this is a known problem for Fantomas.

The tl;dr is that the comment is assigned to the parent node (the entire let binding) instead of the last child in front of it (the last clause). This is wrong, yet supremely hard to deal with.
There have been numerous reports of this problem, we are aware of it and perhaps I should make a general write up why this so hard to figure out.

I would be happy to collaborate on this problem if you are up for it.
Who knows, maybe we can set a step in the right direction.
Let me know if you are interested.

[github-actions]

🤖 This is an automated response from Repo Assist.

I've investigated this bug and submitted a fix.

Root cause: when a trailing comment appears after the last match clause, its range falls outside ExprMatchNode.Range. However, includeTrivia extends Oak.Range to cover all code comments, so the trivia-attachment algorithm falls back to attaching the comment as ContentAfter on ModuleOrNamespaceNode (indent = 0) rather than the match expression — causing the comment to appear at column 0.

Fix: before falling back to the module-level node, simpleTriviaToTriviaInstruction (in Trivia.fs) now checks for a shallower structural node (ExprMatchNode) that ends on the line immediately above the comment and whose start column matches. A shallowest-first traversal ensures the right node is found rather than a deeply nested child token. A secondary fix in addFinalNewline skips structural RestoreAtColumn/RestoreIndent events when checking whether a trailing newline was already emitted.

A draft PR has been submitted with a regression test. All 2742 existing tests still pass.

Generated by Repo Assist · ◷

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@d1d884596e62351dd652ae78465885dd32f0dd7d