Static optimization when ^T struct loses struct #3514

Issue created from fantomas-online

Code

let inline f (x: ^T) = 0 when ^T struct = 1

Result

let inline f (x: ^T) = 0 when ^T = 1

Problem description

A static optimization constraint when ^T struct loses its struct, and the result does not parse: when ^T = 1.

when ^T : int = 1, the other kind of constraint (StaticOptimizationConstraint.WhenTyparTyconEqualsTycon), keeps its type. Static optimizations are meant for FSharp.Core, so this is unlikely to reach anyone else, but the Oak has a node for this case (StaticOptimizationConstraint.WhenTyparIsStruct) and it prints only the type parameter.

Fails on 8.0.6, 7.0.5 and main.

Found while writing a snapshot case for that union case in #3512.

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 2026-10-02T18:44:31Z - 8bf81a1

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.}