Nullness issue - constructor and properties of the DU root, when consumed in C#, do not show nullability of fields

[T-Gro]

Issue description

Reference type DU can be optimized into a single root class with fields.
Either when it is a single-case DU.
Or when it uses null as true value for the nullary cases.

In both scenarios, C# consumers do not see field nullability in the generated constructor.

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

Windows (Default)

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

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

.NET Runtime/SDK version

No response

Reproducible code snippet and actual behavior

[<NoComparison;NoEquality>]
type SingleCaseDu = SingleCaseItIs of nullableString:(string|null)

[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
[<NoEquality; NoComparison>]
type NonGenericNullAsTrueValue = 
    | MyNone
    | MySome of nullableString:(string|null)

Possible workarounds

No response