Nullness issue - Incorrect NullableAttribute on records

[pchalamet]

Issue description

C# (with Nullable enabled) complains about non-nullable arguments when using F# records (warning CS8625: Cannot convert null literal to non-nullable reference type.).

This is probably due to missing attributes on generated properties and constructors:

• NullableAttribute(2) is missing on ctor for nullable params
• NullableAttribute(2) is missing on generated properties (also setter when using [<CLIMutable>]

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

macOS

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

.net sdk 9.0.100

Reproducible code snippet and actual behavior

Considering following type:

type NRTCollection = { Name: string | null }

Several problems can be observed:

var coll = new NRTCollection(null); // `warning CS8625: Cannot convert null literal to non-nullable reference type.`
Console.WriteLine(coll.Name.ToString()); // C# shall warn here since Name can be null - but it's not

Looking at the emitted metadata for the F# type:

[Serializable]
[CompilationMapping(SourceConstructFlags.RecordType)]
[NullableContext(1)]
public sealed class NRTCollection : IEquatable<NRTCollection>, IStructuralEquatable, IComparable<NRTCollection>, IComparable, IStructuralComparable
{
	[CompilerGenerated]
	[DebuggerBrowsable(/*Could not decode attribute arguments.*/)]
	[System.Runtime.CompilerServices.Nullable(2)]
	internal string Name@;

	[CompilationMapping(SourceConstructFlags.Field, 0)]
	public string Name
	{
		[CompilerGenerated]
		[DebuggerNonUserCode]
		get
		{
			return Name@;
		}
	}

	[System.Diagnostics.CodeAnalysis.DynamicDependency(System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.PublicFields | System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.NonPublicFields | System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.PublicProperties | System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.NonPublicProperties, typeof(NRTCollection))]
	public NRTCollection(string name)
	{
		Name@ = name;
	}

I can see that only the backing field Name@ has the Nullable(2) - this is fine but useless as value is accessed through property.

But:

• Constructor parameter name has no attribute
• Property Name has no attribute

This is probably the reason why C# compiler is complaining for records.

if [<CLIMutable>] is set on the record, it's also expected to have the setter annotated with Nullable(2) - this is not the case as of now.

Possible workarounds

None as of now.

[pchalamet]

This is probably fixed by #18079
Feel free to close if that's the case.

[T-Gro]

Indeed, this was solved by the same PR.
Properties and constructors were lacking metadata, and the default kicked in (which for F# generated types is non-nullable by default).