Nullness issue - units of measure don't work with nullables

[Lanayx]

Issue description

I expect string-based measure types (for example using Fsharp.UMX library) to work with null, however it doesn't work.
Linked issue: fsprojects/FSharp.UMX#33

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 10.0.202

.NET Runtime/SDK version

No response

Reproducible code snippet and actual behavior

module Program

[<MeasureAnnotatedAbbreviation>] type string<[<Measure>] 'm> = string
[<Measure>] type private test
type TestType = string<test>

[<EntryPoint>]
let main _ = 
    let x: (TestType | null) = Unchecked.defaultof<TestType>
    0

Produces error
Error FS0043 : A generic construct requires that the type 'TestType' have reference semantics, but it does not, i.e. it is a struct

Possible workarounds

There is no workaround