Potential Set equality regression

Hello, I believe I may have a encountered a quite recent regression with Sets.

Repro steps

Create script:

#r "nuget: NUnit, 3.13.3"

open System.Collections.Generic
open NUnit.Framework

let a : int Set = set (HashSet(0))
Assert.AreEqual(Set.empty, a)

Expected behaviour

Running this with the 7.0.102 SDK works out fine. No exception is thrown.

Actual behaviour

When I run this with my latest local compiler, I got:

dotnet C:\Users\nojaf\Projects\fsharp\artifacts\bin\fsi\Debug\net7.0\fsi.dll .\set-problem.fsx

NUnit.Framework.AssertionException:   Expected is <Microsoft.FSharp.Collections.FSharpSet`1[System.IComparable]>, actual is <Microsoft.FSharp.Collections.FSharpSet`1[System.Int32]>

   at NUnit.Framework.Assert.ReportFailure(String message)
   at NUnit.Framework.Assert.ReportFailure(ConstraintResult result, String message, Object[] args)
   at NUnit.Framework.Assert.That[TActual](TActual actual, IResolveConstraint expression, String message, Object[] args)
   at NUnit.Framework.Assert.AreEqual(Object expected, Object actual)
   at <StartupCode$FSI_0002>.$FSI_0002.main@() in C:\Users\nojaf\Projects\set-problem.fsx:line 7
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)

Related information

@T-Gro maybe this is related to https://github.com/dotnet/fsharp/commit/a9c194aac6ef8951ad0526aad237207838a8351f but that is a wild guess at this point.

Provide any related information (optional):

• Operating system: Windows 11
• .NET Runtime kind: dotnet 7
• Editing Tools (e.g. Visual Studio Version, Visual Studio)

[charlesroddie]

Intuitively, I wouldn’t expect the code to compile, since Set.empty: obj cannot infer the type parameter. If it must compile, I would expect it to be unlikely to evaluate to true, and more likely to evaluate to false, since the type parameter is unlikely to evaluate to int.

Edit: On testing this, Set.empty: obj does compile, which seems like a bug, and the type inside the obj is set<IComparable>. Given that it compiles, the behaviour seems conditionally correct since set<IComparable> is not identical to set<int>.

It would be nice to enable a warning when anything is inferred to be of type obj, or passed into a function as an obj. In the absence of this, we use a test that bans the string “Assert.AreEqual” in .fs files.

[Smaug123]

It would be nice to enable a warning when anything is inferred to be of type obj

😭 I’ve been trying to fix https://github.com/dotnet/fsharp/pull/13298 on-and-off for ages and ages, but I couldn’t get the tests passing.