internal error when implementing interface with auto property in record type (Unexpected definition (AutoProperty...)

[smoothdeveloper]

Under .net framework FSI, I'm getting internal error:

Repro steps

type Foo =
  abstract member X : string with get, set

type FooImpl = 
  { name: string }
  interface Foo with
    member val X = "" with get, set

Actual behavior

error FS0073: internal error: Unexpected definition (AutoProperty
   ([], false, X, None, PropertyGetSet, <fun:_fsyacc_reductions@2322-424>,
    PreXmlDoc ((7,4), FSharp.Compiler.Xml.XmlDocCollector), None,
    \test.fsx (7,17--7,18),
    Const
      (String
         ("", Regular,
          \test.fsx (7,19--7,21)),
       \test.fsx (7,19--7,21)),
    Some
      \test.fsx (7,22--7,26),
    Some
      \test.fsx (7,22--7,35),
    \test.fsx (7,4--7,21)),
 ContainerInfo
  (Parent FSI_0002,
   Some
     (MemberOrValContainerInfo
        (FooImpl,
         Some
           (Foo,
            SlotImplSet
              ([RequiredSlot (FSMeth (<TcGlobals>, Foo, X, None), false);
                RequiredSlot (FSMeth (<TcGlobals>, Foo, X, None), false)],
               map
                 [("get_X",
                   [RequiredSlot (FSMeth (<TcGlobals>, Foo, X, None), false)]);
                  ("set_X",
                   [RequiredSlot (FSMeth (<TcGlobals>, Foo, X, None), false)])],
               [], [FSProp (<TcGlobals>, Foo, Some X, Some X)])), None,
         NoSafeInitInfo, []))))

Expected behavior

Compiles.

Known workarounds

To not use a record.

Related information

• Windows 11
• .NET Framework 4.8 FSI
• VS 2022 17.2.6

[vzarytovskii]

@nojaf should this also be fixed by https://github.com/dotnet/fsharp/pull/13664 or was it only for exception?

Update: probably not, since the change is specific to Exception, and this one is autoprop

[vzarytovskii]

type Foo =
    abstract member X : string with get, set

type FooImpl = 
    interface Foo with
      member val X = "" with get, set

Also fails but for a different reason:

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

stdin(6,22): error FS0971: Undefined value 'X@'

But this code is weird regardless, since FooImpl doesn't have constructor and won't be able to set the underlying X@ to the default value.

[vzarytovskii]

This works, though:

type Foo =
    abstract member X : string with get, set

type FooImpl() = 
    interface Foo with
      member val X = "" with get, set

[vzarytovskii]

Interesting, it also fails on sharplab, which uses 10 months old FCS. It seems bug is there for some time and unrelated to Florian's changes.

[vzarytovskii]

So, we've checked it, and it's broken since at least 3.1.412, not something we introduced recently. We should probably emit different error if we try to have autoproperties on constructor-less types, since we don't generate implicit constructors and I guess it makes no sense having auto-properties on such types. And we should fix the case for the records, I guess?

cc @dsyme @0101

[edgarfgp]

@vzarytovskii Any decision here ? . Happy to help with error reporting

[edgarfgp]

The internal error reported in this issue has been fixed at some point in F# 8 likely by #15343. We have now an error an error message This declaration element is not permitted in an augmentation.

We could close this issue and raise separate issues for :

• Raise a new error if we try to have auto properties on constructor-less types,
• Fix the case for the records supporting auto properties

[edgarfgp]

@vzarytovskii This can be closed