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Nullness issue - Option.toObj infers input to be an option of a nullable type #17727

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@kerams

Description

@kerams

Issue description

When trying to pass a string option value to a null-agnostic method taking string using Option.toObj, the input type gets inferred as string | null option.

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

9.0.100-rc.1.24452.12

Reproducible code snippet and actual behavior

// assembly A, null-agnostic
let work (x: string) = ()

// assembly B, null-aware
let whatever x =
    work (Option.toObj x)

The type of whatever is inferred to be string | null option -> unit, but string option -> unit seems more reasonable.

Possible workarounds

let whatever (x: string option) =
    work (Option.toObj x)

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    Area-NullnessIssues related to handling of Nullable Reference TypesBugImpact-Low(Internal MS Team use only) Describes an issue with limited impact on existing code.help wanted

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