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[WIP] Copilot Request#19503

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copilot/fix-9141961-29048891-e5251906-967f-417d-825d-702105d947cd
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[WIP] Copilot Request#19503
Copilot wants to merge 1 commit intomainfrom
copilot/fix-9141961-29048891-e5251906-967f-417d-825d-702105d947cd

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Copilot AI commented Mar 26, 2026

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

This section details on the original issue you should resolve

<issue_title>No "implementation missing" error for abstract override</issue_title>
<issue_description>A simplified repro of what has actually failed at runtime with no errors reported by F# compiler:

namespace CSharpLib

{
  public abstract class AbstractClass

  {
    public abstract override string ToString();

  }

}

open CSharpLib



type T() =
    inherit AbstractClass()



[<EntryPoint>]

let hello _ =
    let t = T()
    printfn "Hello %O" t
    0

No errors reported by FCS:

Screenshot 2019-10-28 at 13 14 39

After successful build this code fails at runtime:

"C:\Program Files\dotnet\dotnet.exe"
    
C:/Users/Eugene.Auduchinok/RiderProjects/NewAbstractDefaultMember/NewAbstractDefaultMember/bin/Debug/netcoreapp2.0/NewAbstractDefaultMember.dll
Unhandled Exception: System.TypeLoadException: Method 'ToString' in type 'T' from assembly 'NewAbstractDefaultMember, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation.
   at NewAbstractDefaultMember.hello(String[] name)

Process finished with exit code -532,462,766.

```</issue_description>

<agent_instructions>Fix this issue. Instructions:

Read ALL comments on this issue — they contain repro steps, root cause analysis, and prior fix attempts
Add regression tests from the issue description AND every distinct repro in the comments — each becomes a separate test case
Add more tests if they help troubleshoot or verify edge cases
Implement the fix, add release notes 
Run the expert-review skill after the fix and react on its feedback
Run all related tests to verify no regressions</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@dsyme</author><body>
> I didn't even know abstract override existed and I'm wondering how it's supposed to work. 

Likewise, what even is this thing?

</body></comment_new>
</comments>

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Copilot AI requested a review from T-Gro March 26, 2026 22:10
@T-Gro
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T-Gro commented Mar 27, 2026

@copilot : Try again

@T-Gro
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T-Gro commented Mar 27, 2026

@copilot : Try again.

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@T-Gro T-Gro Awaiting requested review from T-Gro

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