Ran into this error today in a F# project:
FSC : error FS0226: The file extension of ‘C:\Users\username\AppData\Local\Programs\Microsoft VS Code\GitHub Copilot’ is not recognized. Source files must have extension .fs, .fsi, .fsx, .fsscript, .ml or .mli.
This took a minute to figure out the cause, and I have no idea how it happened in the first place. The issue was that this Compile line somehow made it into my .fsproj file:
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<ItemGroup>
...
<Compile Include="..\..\..\..\..\..\Users\username\AppData\Local\Programs\Microsoft VS Code\GitHub Copilot" />
...
</ItemGroup>
Removing this fixed it.