The Cabal code base hardly creates CompilerId directly, in almost all cases the change is ignoring the third field. This is where CompilerId is created:

All existing flavours add Nothing, 6x one-line change:

- https://github.com/ghcjs/cabal/commit/ce3c18aedbc4fbf58b150f10690aa5dca6ab0d72#L15R109

Distribution.PackageDescription.Check uses it, but only for checking that the base range is bounded, not with a real compiler, but with buildCompiler with no version:

- https://github.com/ghcjs/cabal/commit/ce3c18aedbc4fbf58b150f10690aa5dca6ab0d72#L2L1240

buildCompilerId uses it (buildCompilerId is never used in Cabal, cabal-install and hackage), a better change would be to find the subcompiler if buildCompilerFlavor is GHCJS, but just adding Nothing makes it do the right thing for all existing compilers:

- https://github.com/ghcjs/cabal/commit/ce3c18aedbc4fbf58b150f10690aa5dca6ab0d72#L1R137

The other changes are the direct intended changes: Updated flag resolution, parsing/printing CompilerId, and getting the shared library filename from a CompilerId