Nullness issue - Spurious(?) 3261 warning with rec module

[g874200]

Issue description

The following module generates a 3261 warning:

module rec M

open System.Collections.Generic

[<Struct>]
type Hole = Hole of string with
    member this.Value =
        let (Hole value) = this in value

type Substitution = Dictionary<Hole,obj>

If you change the first line to module M, there is no warning.

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 10

Reproducible code snippet and actual behavior

No response

Possible workarounds

No response

[g874200]

Actually (unsurprisingly), you don't need module rec -- the following (with an and to make the types mutually recursive) also generates a 3261 warning:

module M

open System.Collections.Generic

[<Struct>]
type Hole = Hole of string with
    member this.Value =
        let (Hole value) = this in value

and Substitution = Dictionary<Hole,obj>