Fix IEquatable<T> nullness contravariance to match C# behavior

[Copilot]

Problem

The F# compiler was emitting nullness warnings when casting non-nullable values to IEquatable<T>, while C# treats IEquatable<T> as contravariant for nullness purposes and suppresses such warnings. This inconsistency created friction when working with F# code that needed to match C# nullness behavior.

open System
let x = ""
let y = x :> IEquatable<string> // Was producing FS3261 nullness warning

Solution

Extended the constraint solver to treat IEquatable<T> type parameters as contravariant for nullness purposes, even though they're not formally marked as contravariant in IL. This matches the behavior implemented in Roslyn for C#.

Changes Made

1. ConstraintSolver.fs: Modified SolveTypeEqualsTypeWithContravarianceEqns to:

   • Accept an optional TyconRef parameter to identify the type constructor
   • Added isIEquatableContravariantForNullness helper function that checks for IEquatable<T>
   • Extended contravariance logic to treat IEquatable<T> parameter 0 as contravariant for nullness
   • Added comprehensive documentation referencing the GitHub issues

2. NullableReferenceTypesTests.fs: Added test case No nullness warning when casting non-nullable to IEquatable to validate the fix

Validation

• ✅ New test passes: No nullness warnings for IEquatable<T> contravariance
• ✅ All 167 existing NullableReferenceTypes tests continue to pass
• ✅ Manual validation confirms compilation without spurious nullness warnings

References

• Fixes Nullness bug when casting a string to an interface #18759
• Related Roslyn issue: Special case nullability for IEquatable<T> roslyn#37187

The fix is minimal and surgical, only affecting the specific IEquatable<T> nullness contravariance case while preserving all other nullness checking behavior.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• 1p7vsblobprodcus324.vsblob.vsassets.io
  • Triggering command: /usr/share/dotnet/dotnet msbuild -v:quiet -restore /home/REDACTED/.packagemanagement/nuget/Projects/112289--a67d0eeb-92ff-4473-9f1c-a88ee35e0deb/Project.fsproj /nologo /t:InteractivePackageManagement (dns block)
• m8dvsblobprodcus37.vsblob.vsassets.io
  • Triggering command: /usr/share/dotnet/dotnet msbuild -v:quiet -restore /home/REDACTED/.packagemanagement/nuget/Projects/112289--a67d0eeb-92ff-4473-9f1c-a88ee35e0deb/Project.fsproj /nologo /t:InteractivePackageManagement (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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This pull request was created as a result of the following prompt from Copilot chat.

Problem

The F# compiler emits a nullness warning when casting a non-nullable value (like "") to IEquatable<string>. C# treats IEquatable<T> as contravariant for nullness, suppressing such warnings. This PR introduces a special case for IEquatable<T> in the F# type checker, so nullness warnings are not emitted for contravariant usage, matching C# behavior.

Implementation

1. ConstraintSolver.fs

   • Extend the logic for contravariant type parameters in SolveTypeEqualsTypeWithContravarianceEqns to also treat the type parameter of IEquatable<T> as contravariant for nullness purposes, even if not marked as such by IL.
   • Add extensive comments explaining why this special case is needed, referencing issue #18759 and the analogous Roslyn issue (#37187).

2. New Test for Nullness

   • Add a test based on the repro in issue #18759 to the Nullness test suite. The test should confirm that no warning is produced when casting a non-nullable value to IEquatable<string>.

References

• Issue being fixed: dotnet/fsharp#18759
• Related Roslyn precedent: dotnet/roslyn#37187

Test Code Example

open System
let x = ""
let y = x :> IEquatable<string> // Should not warn about nullness

Notes

• Extensive commenting should be included in the code to explain the special-casing.
• The new test must fail if any nullness warning is produced for this scenario.

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[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/10.0.100.md	No current pull request URL (#18770) found, please consider adding it

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• 1p7vsblobprodcus324.vsblob.vsassets.io
  • Triggering command: /usr/share/dotnet/dotnet msbuild -v:quiet -restore /home/REDACTED/.packagemanagement/nuget/Projects/172297--5b6a182c-f6a1-4014-a333-5b2c4f96dc72/Project.fsproj /nologo /t:InteractivePackageManagement (dns block)
• m8dvsblobprodcus37.vsblob.vsassets.io
  • Triggering command: /usr/share/dotnet/dotnet msbuild -v:quiet -restore /home/REDACTED/.packagemanagement/nuget/Projects/172297--5b6a182c-f6a1-4014-a333-5b2c4f96dc72/Project.fsproj /nologo /t:InteractivePackageManagement (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

[T-Gro]

Good.
Just add release notes instructions for this in the usual format in docs release notes.
Treat this as "fixed" bug for upcasts to equatable.

[Copilot]

Good.
Just add release notes instructions for this in the usual format in docs release notes.
Treat this as "fixed" bug for upcasts to equatable.

Added release notes entry for the IEquatable nullness fix in the preview.md file, treating it as a fixed bug for upcasts to equatable. Changes committed in 49de3f3.