New case for FS3186: An error occurred while reading the F# metadata node

[dgchurchill]

This is a new case that triggers the "FS3186: An error occurred while reading the F# metadata node..." error previously reported in #592 and #631. It's unlikely to be worth fixing this case itself but I'm reporting it in case it helps identify other corner-cases in the metadata generation.

Repro steps

The error can be triggered by compiling an assembly with a method that has a [<DefaultParameterValue(null)>] attribute on one of its parameters, and then referencing that assembly.

e.g.

Compile this:

namespace Namespace1
open System.Runtime.InteropServices

type Interface1 = 
    abstract Method1 : [<DefaultParameterValue(null)>] arg1 : obj -> unit

Then reference it in F# Interactive:

Microsoft (R) F# Interactive version 14.0.23413.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> #r "Bad.dll";;


unknown(1,1): warning FS3186: An error occurred while reading the F# metadata node at position 0 in table 'itypars' of assembly 'Bad, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. The node had no matching declaration. Please report this warning. You may need to recompile the F# assembly you are using.

--> Referenced 'Bad.dll'

Expected behavior

The compiler should either fail when compiling [<DefaultParameterValue(null)>] or produce correct metadata. However also see #96 -- DefaultParameterValue is not actually supported by the F# compiler. So no one can be using this, and in this case could use the [<Optional>] attribute and deal with Type.Missing anyway.

Related information

Tested using Microsoft (R) F# Compiler version 14.0.23413.0

[dsyme]

Still repros on master

[kurtschelfthout]

I bet #1812 fixes this but I will check.

[kurtschelfthout]

I was wrong; this has nothing to do with DefaultParameterValue per se. I think the compiler infers a type argument because of the null but half-asses it or something along those lines. This also triggers the bug:

namespace Namespace1
open System
open System.Runtime.InteropServices

[<AttributeUsage(AttributeTargets.All, AllowMultiple=true)>]
type FooAttribute(t:obj) =
   inherit System.Attribute()

type Interface1 = 
    abstract Method1 : [<FooAttribute(null)>] arg1 : obj -> unit

So really unrelated to #96, though likely would be hit in the original case (DefaultParameterValue) most often.

[dsyme]

@kurtschelfthout This is an old issue, but the workaround is a simple type annotation DefaultParameterValue(null:obj)

namespace Namespace1
open System.Runtime.InteropServices

type Interface1 = 
    abstract Method1 : [<DefaultParameterValue(null:obj)>] arg1 : obj -> unit

I will close this as a duplicate of #719

[Lanayx]

Hit this issue today with the following commit: fsprojects/pulsar-client-dotnet@4267a82 (SequenceId is UMX type for uint64)
Ignored it as was suggested in #592

[gusty]

I also see it back after compiling and referencing F#+ with VS 2019 v. 16.7.0 Preview 1.0

[cartermp]

I think this should be re-opened since it's clearly happening in multiple codebases

[Shmew]

I'm seeing this when building my library, located in this repo.

[cartermp]

@dsyme any thoughts on how to resolve this?