Multiline AbstractSlot without constraints introduces newline

[MangelMaxime]

When trying to format some code using Fantomas I got an Idempotency warning.

I was able to narrow it to these code:

The code can looks strange but that's because it is a Fable binding

Issue created from fantomas-online

Formatted code

// ts2fable 0.8.0
module rec Glutinum.Fuse

#nowarn "3390" // disable warnings for invalid XML comments

open System
open Fable.Core
open Fable.Core.JS

module Fuse =

    [<AllowNullLiteral>]
    type FuseSortFunctionItem =
        [<EmitIndexer>]
        abstract Item: key: string -> U2<{| ``$``: string |}, ResizeArray<{| ``$``: string; idx: float |}>> with get, set


    [<AllowNullLiteral>]
    [<Global>]
    type FuseOptionKeyObject [<ParamObject; Emit("$0")>] (name: U2<string, ResizeArray<string>>, weight: float) =
        member val name: U2<string, ResizeArray<string>> = jsNative with get, set
        member val weight: float = jsNative with get, set

Reformatted code

// ts2fable 0.8.0
module rec Glutinum.Fuse

#nowarn "3390" // disable warnings for invalid XML comments

open System
open Fable.Core
open Fable.Core.JS

module Fuse =

    [<AllowNullLiteral>]
    type FuseSortFunctionItem =
        [<EmitIndexer>]
        abstract Item: key: string -> U2<{| ``$``: string |}, ResizeArray<{| ``$``: string; idx: float |}>> with get, set



    [<AllowNullLiteral>]
    [<Global>]
    type FuseOptionKeyObject [<ParamObject; Emit("$0")>] (name: U2<string, ResizeArray<string>>, weight: float) =
        member val name: U2<string, ResizeArray<string>> = jsNative with get, set
        member val weight: float = jsNative with get, set

Problem description

Fantomas was not able to produce the same code after reformatting the result.

Extra information

• The formatted result breaks by code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas master branch at 2022-03-29T09:12:14Z - 8f2ab14

    { config with
                MaxArrayOrListWidth = 5
                MultilineBlockBracketsOnSameColumn = true }

_{Did you know that you can ignore files when formatting from fantomas-tool or the FAKE targets by using a .fantomasignore file?}

[baronfel]

This took me a minute to spot, but the warning is coming from the addition of another newline in between the definitions of FuseSortFunctionItem and FuseOptionKeyObject.

[nojaf]

Hello Maxime, thank you for reporting this issue.
I was able to reduce the problem to

[<Test>]
let ``multiline abstract member without constraints, 2175`` () =
    formatSourceString
        false
        """
    type FuseSortFunctionItem =
        abstract Item: key: string -> U2<{| ``$``: string |}, ResizeArray<{| ``$``: string; idx: float |}>> with get, set
        abstract X : int
"""
        { config with MaxLineLength = 60 }
    |> prepend newline
    |> should
        equal
        """
type FuseSortFunctionItem =
    abstract Item:
        key: string ->
            U2<{| ``$``: string |}, ResizeArray<{| ``$``: string
                                                   idx: float |}>> with get, set

    abstract X: int
"""

The AbstractSlot does not have any constraints but is already crossing the max line length.
This will trigger:

fantomas/src/Fantomas/CodePrinter.fs

Lines 4829 to 4838 in 7373a06

	genPreXmlDoc px
	+> genAttributes astContext ats
	+> opt sepSpace ao genAccess
	+> genMemberFlags mf
	+> !-s
	+> genTypeParamPostfix astContext tds
	+> ifElse hasGenerics sepColonWithSpacesFixed sepColon
	+> autoIndentAndNlnIfExpressionExceedsPageWidth (genTypeList astContext namedArgs)
	-- genPropertyKind (not isFunctionProperty) mf.MemberKind
	+> autoIndentAndNlnIfExpressionExceedsPageWidth (genConstraints astContext t vi)

to add a new line.

It can be resolved by adding an additional check like:

        +> onlyIf
            (match t with
             | TWithGlobalConstraints _ -> true
             | _ -> false)
            autoIndentAndNlnIfExpressionExceedsPageWidth
            (genConstraints astContext t vi)

Are you interested in submitting a PR for this?

[MangelMaxime]

Hello @nojaf,

thank you for the clear explanations.

Are you interested in submitting a PR for this?

I don't think I will have the time to look into the issue as I have a lot of stuff on my side to handle already. And learning about Fantomas internal, will probably takes me some times ^^ sorry.