Motivation/Use case: * Since gcc/g++ intentionally does not have `-Weverything`, there is a number of explicit `-W...` flags one might wish to specify explicitly. Fair enough.
* Additional `-W...` flags are introduced in new gcc/g++ versions, which check for new potential code smells, possibly related to later language standards. That's great (thanks!). * Not all platforms run the latest gcc/g++ compiler version. So build tools need to deal with a range of versions. Taking these three points together, we have a problem. The build tool wishes to specify `-Wshiny-new-warning` but may only do so if the gcc/g++ compiler version is above some version X. This is difficult for several reasons: * The documentation does not clearly specify the 1st compiler version that supports each error/warning command line flag. One has to manually backtrack through the release notes for all the compiler versions. In fact the documentation also doesn't make it easy to see which flags are covered by other flags - one has to read through the documentation, one flag at a time, and extract the information from the English text. Some form of a more structured table would have been awesome: 1st compiler version supporting the flag, the list of other flags such as `-Wall` that imply the flag, maybe a list of related flags such as `-f...`. But that aside... * Even if we know which compiler version to check for, this quickly becomes a PITA to maintain in the build tool, given the large number of compiler versions in use in the wild. Having many versions means the compiler has been actively progressing at a fast rate, which is great! But it makes it nasty to deal with providing the correct warning flags for the correct compiler version. This is (mostly) solved with the new proposed flag: the build tool would simply list all the desired warning flags, which would be silently ignored by older compiler versions. Typos are a potential issue. One would have to run a "recent" version without the `-Wno-unknown-warnings` to catch such typos; that would be rare, and is simple enough to do (and automate in the build tool if desired). Another obvious issue is that the new `-Wno-unknown-warnings` flag would only be available starting in a future compiler version. So build tools would have to deal with the problem for a long time before the flag would provide its full benefits. Barring having a working time machine ;-) However, it is much simpler for a build tool to do a single test of whether or not the compiler version supports the proposed new flag, than to deal with the full problem of multiple versions. If the new flag is not supported, the build tool could fall back to using a conservative set of warning flags. Also, as time goes by, older compiler versions would eventually be phased out, so even this test would be (eventually) removed. Sure, it would take a decade or more, but: as the saying goes, "The best time to plant a tree is 20 years ago; the second best time is today". Thanks, Oren Ben-Kiki
