[Bug] Struct constructor parameter renamed when name matches field (causes FS0039)

[itsdanieldk]

Issue created from fantomas-online

Code

 module Bug
 
 type Cont = SuccessCont
 
 [<Struct>]
 type ContStackFrame =
     val Cont: Cont
     new cont = { Cont = cont }

Result

module Bug

type Cont = SuccessCont

[<Struct>]
type ContStackFrame =
    val Cont: Cont
    new newcont = { Cont = cont }

Problem description

Fantomas renames the struct constructor parameter cont to newcont by prepending new to the identifier. The record initialization { Cont = cont } still references the original parameter name, so the formatted code fails to compile with:

error FS0039: The value or constructor 'cont' is not defined.

This is also an idempotency issue. Each subsequent format pass prepends another new:

Pass 1: new cont → new newcont
Pass 2: new newcont → new newnewcont
Pass 3: new newnewcont → new newnewnewcont

This appears to happen when the constructor parameter name matches the field name (differing only in case): val Cont: Cont + new cont = .... Fantomas seems to confuse the parameter with the new keyword and merges them.

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

Default Fantomas configuration

[github-actions]

🤖 This is an automated response from Repo Assist.

I've investigated this issue and confirmed the root cause. A fix PR has been submitted.

Root cause: In ASTTransformer.fs, the pattern match for MemberDefn.ExplicitCtor only recognized parenthesized constructor arguments — SynArgPats.Pats [ SynPat.Paren _ as pat ]. When the constructor argument is unparenthesized (new cont = ...), the match failed and the binding fell through to mkBinding. That function created a BindingNode with "new" in both LeadingKeyword and FunctionName. The "new" function-name branch in genBinding suppresses the space before the parameter, so the two tokens concatenated to "newcont".

Fix: Two changes:

1. ASTTransformer.fs: Accept any single argument pattern (not just parenthesized) in the ExplicitCtor match.
2. CodePrinter.fs: Always use a space before non-paren patterns (bare identifiers always need a space after new, unlike parenthesized forms which respect SpaceBeforeClassConstructor).

The fix is in branch repo-assist/fix-issue-3349-struct-ctor-unparenthesized; a draft PR has been submitted. All 2837 existing tests continue to pass, and 2 regression tests were added.

Generated by 🌈 Repo Assist, see workflow run. Learn more.

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@97143ac59cb3a13ef2a77581f929f06719c7402a

[nojaf]

Available in https://github.com/fsprojects/fantomas/releases/tag/v8.0.0-alpha-010

[itsdanieldk]

Thanks @nojaf 😄