open System
open System.Collections.Generic
open System.Linq

module Extensions =

    type ILookup<'Key, 'Value when 'Key : not null> with

        static member Empty = Seq.empty<KeyValuePair<'Key, 'Value>>.ToLookup(_.Key, _.Value)

        member lookup.RemoveKey (key : 'Key) =
            lookup
            |> Seq.where (fun g -> 'Key.Equals(g.Key, key))
            |> Seq.collect (fun g -> g |> Seq.map (fun v -> KeyValuePair(g.Key, v)))
            |> _.ToLookup(_.Key, _.Value)

displays:

• error FS0957: One or more of the declared type parameters for this type extension have a missing or wrong type constraint not matching the original type constraints on 'ILookup<_,_>'
• error FS0340: The signature and implementation are not compatible because the declaration of the type parameter 'Key' requires a constraint of the form 'Key: not null
  while the required type constraint is present

Repro steps

use the uploaded class library project
WrongNonNullConditionRepro.zip

Expected behavior

No error

Actual behavior

• error FS0957: One or more of the declared type parameters for this type extension have a missing or wrong type constraint not matching the original type constraints on 'ILookup<_,_>'
• error FS0340: The signature and implementation are not compatible because the declaration of the type parameter 'Key' requires a constraint of the form 'Key: not null

Known workarounds

No

Related information

Provide any related information (optional):

• .NET 8
• Visual Studio 2022 17.13.1