DotLambda underscore cannot have space after unit

[nojaf]

Issue created from fantomas-online

Code

"yow" |> _.Substring(0, 16).ToLower()

Result

"yow" |> _.Substring(0, 16).ToLower()

Problem description

Code broke:

  "yow" |> _.Substring(0, 16).ToLower ();;
  ---------^^^^^^^^^^^^^^^^^^^^^^^^^^

/Users/nojaf/Projects/stdin(1,10): error FS3584: Shorthand lambda syntax is only supported for atomic expressions, such as method, property, field or indexer on the implied '_' argument. For example: 'let f = _.Length'.

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.
• I would like a release if this problem is solved.

Options

Fantomas main branch at 2026-04-27T09:49:00Z - 1277944

    { config with
                SpaceBeforeUppercaseInvocation = true }

_{Did you know that you can ignore files when formatting by using a .fantomasignore file?
PS: It's unlikely that someone else will solve your specific issue, as it's something that you have a personal stake in.}

[github-actions]

🤖 This is an automated response from Repo Assist.

I've investigated this issue and submitted a fix as a draft PR.

Root cause: Inside genDotLambdaExpr, an Expr.Chain node (e.g. Substring(0, 16).ToLower()) fell through to genExpr expr. The chain renderer adds a space before the last link's parens via sepSpaceBeforeParenInFuncInvocation when isLastLink = true and SpaceBeforeUppercaseInvocation = true, inserting the unwanted space before ToLower().

Fix: Added an Expr.Chain case in genDotLambdaExpr that renders each link without the space-before-parens logic, keeping DotLambda expressions atomic as required by the F# compiler. All 2841 tests pass.

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