Issue description

I believe that Async.SwitchToContext should take a parameter of type System.Threading.SynchronizationContext | null. Examining the function body, you can see that it treats null as a possible argument and handles it normally (without throwing an exception).

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

net9.0

Reproducible code snippet and actual behavior

No response

Possible workarounds

One can write a wrapper function which when given null calls Async.SwitchToThreadPool, otherwise calls SwitchToContext with the non-null syncContext.