Default use of implicit conversion creates non-working code (using Pulumi libraries)

The code generated by the new (12.0.1.0 compiler for F# 6) which implicitly uses op_Implicit is generating code that doesn’t work the same way that calling op_Implict (or using C#'s implicits) does.

This doesn’t work

    let conditionImplicit =
        Cdn.Inputs.DeliveryRuleRequestSchemeConditionArgs(
            Name = "RequestScheme1",
            Parameters =
                Cdn.Inputs.RequestSchemeMatchConditionParametersArgs(
                    MatchValues = matchValues,
                    Operator = Input.op_Implicit "Equal",
                    OdataType = Input.op_Implicit "#Microsoft.Azure.Cdn.Models.DeliveryRuleRequestSchemeConditionParameters"
                )
        )

Not using implicit conversions does work

    let conditionExplicit =
        Cdn.Inputs.DeliveryRuleRequestSchemeConditionArgs(
            Name = "RequestScheme2",
            Parameters =
                Input.op_Implicit (Cdn.Inputs.RequestSchemeMatchConditionParametersArgs(
                    MatchValues = matchValues,
                    Operator = Input.op_Implicit "Equal",
                    OdataType = Input.op_Implicit "#Microsoft.Azure.Cdn.Models.DeliveryRuleRequestSchemeConditionParameters"
                ))
        )

C#'s implicit conversions do work

            var conditionImplicit =
                new DeliveryRuleRequestSchemeConditionArgs {
                    Name = "RequestScheme1",
                    Parameters =
                        new RequestSchemeMatchConditionParametersArgs {
                            MatchValues = matchValues,
                            Operator = "Equal",
                            OdataType = "#Microsoft.Azure.Cdn.Models.DeliveryRuleRequestSchemeConditionParameters"
                        }
                };

The repro case is at https://github.com/marklam/ImplicitsProblem

It looks like the code generation (as disassembled with ILspy) is different for the F# implicit case. This means that when this code is used within a Pulumi app, errors like https://github.com/pulumi/pulumi-azure-native/issues/1569 occur.

It seems like this is being produced by the implicit calls:

 call class [Pulumi]Pulumi.Input`1<!0> class [Pulumi]Pulumi.Input`1<class [Pulumi.AzureNative]Pulumi.AzureNative.Cdn.Inputs.RequestSchemeMatchConditionParametersArgs>::op_Implicit(!0)  

instead of

newobj instance void [Pulumi.AzureNative]Pulumi.AzureNative.Cdn.Inputs.DeliveryRuleRequestSchemeConditionArgs::.ctor()

Known workarounds

Enable warnings for implicit conversions, and use the old way (call op_Implicit yourself)

<WarnOn>3388;3391;3395</WarnOn>

Related information

Provide any related information (optional):

• Windows 10
• Visual Studio 2022
• F# 6
• .NET Runtime 3.1

[NinoFloris]

Thanks!

I’ve minimized the repro further and analyzed the IL.

type Input<'T>(_v: 'T) =
    static member op_Implicit(value: 'T): Input<'T> = Input<'T>(value)

type OtherArgs() =
    member val Name: string = Unchecked.defaultof<_> with get,set
type SomeArgs() =
    member val OtherArgs: Input<OtherArgs> = Unchecked.defaultof<_> with get, set
    
let test() =
    SomeArgs(OtherArgs = OtherArgs(Name = "test"))

The il of test looks like this:

    .locals init (
      [0] class Program/SomeArgs V_0,
      [1] class Program/OtherArgs V_1
    )

    // [18 5 - 18 51]
    IL_0000: newobj       instance void Program/SomeArgs::.ctor()
    IL_0005: stloc.0      // V_0
    IL_0006: ldloc.0      // V_0
    IL_0007: newobj       instance void Program/OtherArgs::.ctor()
    IL_000c: call         class Program/Input`1<!0/*class Program/OtherArgs*/> class Program/Input`1<class Program/OtherArgs>::op_Implicit(!0/*class Program/OtherArgs*/)
    IL_0011: stloc.1      // V_1
    IL_0012: ldloc.1      // V_1
    IL_0013: ldstr        "test"
    IL_0018: callvirt     instance void Program/OtherArgs::set_Name(string)
    IL_001d: nop
    IL_001e: ldloc.1      // V_1
    IL_001f: call         class Program/Input`1<!0/*class Program/OtherArgs*/> class Program/Input`1<class Program/OtherArgs>::op_Implicit(!0/*class Program/OtherArgs*/)
    IL_0024: callvirt     instance void Program/SomeArgs::set_OtherArgs(class Program/Input`1<class Program/OtherArgs>)
    IL_0029: nop
    IL_002a: ldloc.0      // V_0
    IL_002b: ret

Following the lines, starting from IL_0011 the compiler went astray:

1. It emitted a stloc.1 for the result of the implicit conversion from OtherArgs to Input<OtherArgs>. Meaning it now holds a value of Program/Input`1<!0/class Program/OtherArgs/> in local [1] which has the expected type Program/OtherArgs, this is obviously not correct.
2. Next we load the same Input`1 again onto the stack.
3. We load the constant string “test” I used for Name onto the stack.
4. Then at IL_0018 it uses these two stack entries, the Input`1 as the supposed OtherArgs instance and the string as the first argument to set_Name.

Pushing this all into ilverify confirms my conclusion:

[IL]: Error [StackUnexpected]: [FsImplicitTest.dll : .Program::test()][offset 0x00000011][found ref '[FsImplicitTest]Program+Input`1<Program+OtherArgs>'][expected ref '[FsImplicitTest]Program+OtherArgs'] Unexpected type on the stack.

Now at this point if this was a virtual property the runtime would have to look at the instance reference it has on the stack to run set_Name for (in this case a reference to an Input). It would get the method table from the object header and try to find set_Name which doesn’t exist on Input. The runtime would then throw some invalid program exception killing the process, which would have made this a bit easier 😃

Here though, because it’s a non virtual method, the instructions were even inlined (and the F# compiler might also do this if Pulumi’s Input class was written in F#). As a result this runtime work does not happen and code just overwrote what was there (in whatever instance reference given) at the offset the backing field for Name would normally live. For Input this offset corresponds with the underlying/wrapped value. And indeed navigating the value in the debugger shows that SomeArgs.OtherArgs now directly points to an Input storing Name instead of OtherArgs. Heap corruption!

Fix is in https://github.com/dotnet/fsharp/pull/13673

[NinoFloris]

Not sure, might have made it for rc1 but otherwise rc2 maybe?