Issue description

The following code triggers FS3261:

let blip =
    match true with
    | true -> null
    | false -> "test"

with blip being inferred as type string. The above should reasonably be inferred as string | null, and I believe the equivalent of the above using if / then / else does indeed do that.

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 9

Reproducible code snippet and actual behavior

let blip =
    match true with
    | true -> null
    | false -> "test"

Possible workarounds

Add a type annotation on blip e.g. blip : string | null.