Marking external headers with -isystem is a common strategy for dealing with warning suppression for files outside your control. This can be a mistake as it can suppress warnings from misuse such as conversion in std:: optional, but such are the tradeoffs for false positives. 

If module interfaces are built as part of a project do we need mechanisms to suppress warnings? Standard libraries use pragmas to mark themselves, but most libraries will and should not? 

Thoughts raised due to https://lucisqr.substack.com/p/the-warnings-you-never-saw -- which I have issues with and have not completely verified.