Nullness issue - WarningAsError doesn't work for 3261

[Lanayx]

Issue description

WarningAsError doesn't have effect in certain cases

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

.NET SDK 9.0.100

Reproducible code snippet and actual behavior

1. Clone Oxpecker repository, observe this line, nulness warnings are set as errors
2. Go to this line, change (rawRangeHeader[0] |> string) to just rawRangeHeader[0]
3. Build the project
4. Observer the warning that is not an error

Possible workarounds

Ignore problem

[abonie]

Hi @Lanayx , is it possible for you to provide a minimal repro (for our convenience)?

[Lanayx]

@abonie it looks like this code is sufficient

let test (s: string | null): string =
    s

[Lanayx]

Oops, as I checked, it appeared that WarningsAsErrors was defined twice and the second one overrode the correct one, sorry for that and thanks for checking