Improve diagnostic: member val in interface

[bleis-tift]

Automatically implemented property occurs internal error.

Repro steps

// fsi
> type I =
  abstract member X: int with get, set

type R =
  { x: int }
  interface I with
    member val X = 0 with get, set;;

      member val X = 0 with get, set;;
  -----------^^^^^^^^^

stdin(7,12): error FS0073: internal error: Unexpected definition

Expected behavior

No compile errors.

Actual behavior

Internal error.

Known workarounds

// fsi
> type I =
  abstract member X: int with get, set

type R =
  { mutable x: int }
  interface I with
    member this.X with get() = this.x and set(v) = this.x <- v;;
type I =
  interface
    abstract member X : int
    abstract member X : int with set
  end
type R =
  {mutable x: int;}
  with
    interface I
  end

> 

Related information

• Operating system: Windows 10
• Visual F# Tools: 15.4.1.17052902
• Editing Tools: Visual Studio 2017

[forki]

This error happens in TypeChecker.fs in TcMutRecBindings_Phase2A_CreateRecursiveValuesAndCheckArgumentPatterns.

There is a match classMemberDef, containerInfo with which doesn't match SynMemberDefn.AutoProperty and just fails. @KevinRansom I don't know to implement that matching case.

[dsyme]

@forki @bleis-tift Just to note that the member val isn't allowed in an interface implementation on a record or union type (because it implies a let definition in the type, which is not allowed). So this is "just" a poor error message

[realvictorprm]

Just ran into that one:

error FS0073: internal error: Unexpected definition (AutoProperty
   ([],false,X,None,Member,<fun:_fsyacc_reductions@2046-415>,
    PreXmlDoc ((20,24),FSharp.Compiler.Ast+XmlDocCollector),None,
    Const (Int32 10,_.fs (20,25--20,27) IsSynthetic=false),None,
    _.fs (20,8--20,27) IsSynthetic=false),
 ContainerInfo
  (Parent _,
   Some
     (MemberOrValContainerInfo
        (DogData,
         Some
           (IPosition,
            SlotImplSet
              ([RequiredSlot (FSMeth (<TcGlobals>,IPosition,X,None),false);
                RequiredSlot (FSMeth (<TcGlobals>,IPosition,Y,None),false)],
               map
                 [("get_X",
                   [RequiredSlot (FSMeth (<TcGlobals>,IPosition,X,None),false)]);
                  ("get_Y",
                   [RequiredSlot (FSMeth (<TcGlobals>,IPosition,Y,None),false)])],
               [],
               [FSProp (<TcGlobals>,IPosition,Some Y,None);
                FSProp (<TcGlobals>,IPosition,Some X,None)])),None,
         NoSafeInitInfo,[]))))

Well even if it's a bad error message it still is a bad error message which really needs to be fixed.

[edgarfgp]

This seems to be fixed in F# 8 as it not longer throws an internal error but now shows This declaration element is not permitted in an augmentation cc @vzarytovskii