Nullness issue - Async.SwitchToContext lacks a null annotation on its parameter

[glchapman]

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.

[T-Gro]

You are correct, thank you for spotting this.

The annotation at

fsharp/src/FSharp.Core/async.fsi

Line 650 in 744aa02

static member SwitchToContext : syncContext:System.Threading.SynchronizationContext -> Async<unit>

must be adjusted, the doc comment is already correct.

[BoundedChenn31]

Looks like this annotation and all compiler directives at CreateSwitchToAsync can be removed? It is called only by SwitchToContext that checks context for null and PostWithTrampoline definitely doesn't handle nulls well.

fsharp/src/FSharp.Core/async.fs

Lines 871 to 876 in 30d9ea7

	#if BUILDING_WITH_LKG || NO_NULLCHECKING_LIB_SUPPORT
	let CreateSwitchToAsync (syncCtxt: SynchronizationContext) =
	#else
	let CreateSwitchToAsync (syncCtxt: SynchronizationContext | null) =
	#endif
	MakeAsyncWithCancelCheck(fun ctxt -> ctxt.PostWithTrampoline syncCtxt ctxt.cont)

[T-Gro]

CreateSwitchToAsync is not part of the public surface (IIRC). If it is not called with a potentially null value anywhere in FSharp.Core, it can be made strict indeed .