[`module rec`] does not initialize let bindings

[xperiandri]

Repro steps

Run reproduction sample
ModuleRecIssueRepro.zip

Notice that in the file AssessmentDataValidated.fs at line 61 Codec value is null!

[<AutoOpen>]
module CodecsExtensions =

    module AsseseeCodecs =

        open System.Net.Mail

        let Codec: Codec<Encoding, Assessee> = // <<----------------- null is here!
            ofObjCodec
            <| codec {
                let! lastName = jreq "lastName" (fun a -> a.LastName |> Some)
                and! firstName = jreq "firstName" (fun a -> a.FirstName |> Some)

                and! middleName =
                    jopt "middleName" (fun a -> a.MiddleName |> ValueOption.toOption)

                and! dateOfBirth = jopt "dateOfBirth" (fun a -> a.DateOfBirth |> ValueOption.toOption)
                and! phone = jopt "phone" (fun a -> a.Phone |> ValueOption.toOption)
                and! email = jopt "email" (fun a -> a.Email |> ValueOption.map (fun mail -> mail.Address) |> ValueOption.toOption)

                return {
                    LastName = lastName
                    MiddleName = middleName |> Option.toVOption
                    FirstName = firstName
                    DateOfBirth = dateOfBirth |> Option.toVOption
                    Phone = phone |> Option.toVOption
                    Email = email |> Option.toVOption |> ValueOption.map MailAddress
                }
            }

    type Assessee with

        static member Codec = AsseseeCodecs.Codec

[<RequireQualifiedAccess>]
module Codecs =

    let V1: Codec<Encoding, EventData> =
        let assesseeCodec = Assessee.Codec
        ofObjCodec
        <| codec {
            let! assessee = jreqWith Assessee.Codec "assessee" (fun di -> di.Assessee |> Some)
            let x = { Assessee = assessee(*; DrivingLicense = drivingLicense; Address = address*) }
            return x
        }

Expected behavior

Codec value is codec

Actual behavior

Codec value is null

Known workarounds

Do not use module rec

Related information

• .NET 7
• Visual Studio 17.8 Preview 2.0

[vzarytovskii]

First broken in 5483858 (part of #15343)

[vzarytovskii]

What we generate for binding (roughtly):

.property class [Fleece]Fleece.Codec`4<class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class Domain.Assessee, class Domain.Assessee> Codec()
{
	.custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationMappingAttribute::.ctor(valuetype [FSharp.Core]Microsoft.FSharp.Core.SourceConstructFlags) = (
		01 00 09 00 00 00 00 00
	)
	.get class [Fleece]Fleece.Codec`4<class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class Domain.Assessee, class Domain.Assessee> Domain.Events.AssessmentDataValidated/CodecsExtensions/AsseseeCodecs::get_Codec()
}

.method public specialname static 
	class [Fleece]Fleece.Codec`4<class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class Domain.Assessee, class Domain.Assessee> get_Codec () cil managed 
{
	// Method begins at RVA 0x2ce0
	// Header size: 1
	// Code size: 6 (0x6)
	.maxstack 8

	IL_0000: ldsfld class [Fleece]Fleece.Codec`4<class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class Domain.Assessee, class Domain.Assessee> '<StartupCode$ModuleRecIssueRepro>.$Domain.Events.AssessmentDataValidated'::'Codec@61-2'
	IL_0005: ret
} // end of method AsseseeCodecs::get_Codec

And a field:

.field assembly static initonly class [Fleece]Fleece.Codec`4<class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class [Fleece.SystemTextJson]Fleece.SystemTextJson.Encoding, class Domain.Assessee, class Domain.Assessee> 'Codec@61-2'
.custom instance void [System.Runtime]System.Diagnostics.DebuggerBrowsableAttribute::.ctor(valuetype [System.Runtime]System.Diagnostics.DebuggerBrowsableState) = (
	01 00 00 00 00 00 00 00
)

I couldn't find the initialization code for the field.

[vzarytovskii]

Minimal repro:
modulerecrepro.zip

[T-Gro]

Fixed by #16054