install(EXPORT): Should the integrity check really be fatal?
Should the integrity check in cmExportFileGenerator::GenerateImportedFileCheckLoop really be fatal?
https://gitlab.kitware.com/cmake/cmake/-/blob/60a1ccbd6a913d09d7c0a3f2b104e60699718e07/Source/cmExportFileGenerator.cxx#L1197 produces a FATAL_ERROR if a file that got installed alongside the cmake exports file is not installed on the system. While this can be useful, it is often a non-issue: For example, in Qt 6, this triggers a fatal error if any of the optional widget style plugins [that nothing could link to at build time even if it wanted to] have been removed. Or, in LLVM, it triggers a fatal error if MLIR is not installed when something just uses cmake to check for libLLVM. Of course a possible answer to this is "don't build the bits you don't need then" -- but in the case of distribution packages, this is not always possible (e.g. we certainly want to have all the Qt styles available to our users, but we don't want to force anyone using cmake to link against QtGui to install the GTK style plugin which pulls in hundreds of megabytes of dependencies). For those use cases, it would be preferable to downgrade that FATAL_ERROR to a WARNING - if afterwards, everything succeeds, it's clear that the installation is just "missing" an optional and useless in the context file, but if something fails, the user will still have seen the warning and knows what file he may need to look for.