FSharpCompilerPath Override for Version-Pinned Upgrade Testing

Direct Answer

The FSharpCompilerPath override alone does not prevent incremental build failure when compiler API changes invalidate existing type providers. Incremental caches and type provider metadata are stamped with the compiler version that generated them; overriding the path selects which compiler binary is invoked but does not automatically invalidate version-mismatched artifacts. Similarly, paket.dependencies resolution does not automatically revert NuGet package versions when a compiler override is in place, and F# Interactive session restoration via #load can fail if loaded scripts or type providers were compiled against a different compiler version than the pinned path.

Likely Explanation vs Confirmed Facts

• Likely: The override directs the F# compiler executable, but build caches, object files, and type provider caches may retain references to the previous compiler version. When API surfaces change, the incremental engine may attempt to reuse stale artifacts, causing build failures or type provider load errors.
• Confirmed (from research brief context): Paket.dependencies resolution does not automatically revert NuGet package versions when a compiler override is active. F# Interactive session persistence does not account for compiler-specific type provider assemblies; restart after upgrade can crash or produce unresolved type references.

Steps Needed for This Case

1. Clean the build cache: run dotnet clean or manually remove obj/ and bin/ directories under the project path to discard version-stamped artifacts.
2. Verify the compiler path takes effect: run fsi --version before and after setting FSharpCompilerPath in the .fsharpproj to confirm the override is recognized by the runtime.
3. Rebuild with the overridden path: invoke dotnet build (or the project-specific build command) and observe whether type provider errors reappear.
4. For paket dependency conflicts: run paket resolve --refresh to force re-evaluation of NuGet version constraints, or adjust paket.dependencies constraints if newer versions are fetched.
5. For F# Interactive session recovery after upgrade failure: re-run #load only after ensuring all referenced type provider assemblies are rebuilt against the pinned compiler version.

Missing Diagnostic Detail

To determine whether a full cache invalidation or a paket refresh is the appropriate next step, please share the specific type provider or assembly error message observed when rebuilding with the overridden FSharpCompilerPath. This detail would change the recommended remediation.