Nullness issue - Apparently incorrect 3261 in match arm

[g874200]

Issue description

I'm using dotnet 10.0.107. This code:

open System.Collections.Immutable

let sets (builders: (ImmutableHashSet<int>.Builder | null)[]) = [
    for builder in builders do
        match builder with
        | null -> ()
        | b -> yield b.ToImmutable()
]

generates a 3261 nullness warning on the b.ToImmutable(). The warning goes away if you replace the b with (nonNull b). Shouldn't the null match arm mean there's no need for a nonNull assertion?

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

No response

Possible workarounds

No response