Exception abbreviation loses its right-hand side #3511

Issue created from fantomas-online

Code

exception MyError = System.Exception

Result

exception MyError

Problem description

An exception abbreviation loses its right-hand side: exception MyError = System.Exception formats to exception MyError, which declares a new exception instead of an abbreviation. The result is valid F#, so nothing flags the change.

The same happens in a signature file. It fails on 7.0.5, 8.0.5 and main.

ASTTransformer seems to skip the abbreviated name that SynExceptionDefnRepr carries.

Found while writing tests for exception declarations.

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 2026-10-01T08:59:20Z - 6e286d1

Default Fantomas configuration

_{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.}