Nullness issue - ToString on an integer with UoM takes nullness information from ValueType

[kerams]

Issue description

Similarly with UMX when the underlying type is a reference type, nullness is inherited from Object.ToString, rather than the underlying type, whose overriden ToString is not null.

Choose one or more from the following categories of impact

• [X] 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

9.0.0-preview.7.24405.7

Reproducible code snippet and actual behavior

> (1).ToString();;
val it: string = "1"

> [<Measure>] type id;;
[<Measure>]
type id

> 1<id>.ToString ();;
val it: string | null = "1"

Possible workarounds

Unchecked.nonNull