It is certainly not a feature because it make all infrastructure including just regular old checkout-and-build workflows break for historical versions of the code. It’s so annoying to have to checkout an older version and then have to go disable -Wall -Werror everywhere just to get the damn thing to build.
Keep master clean of any warnings, for sure. But don’t put it straight into the build system defaults.
Just updating a compiler could break workflows for historical versions of the code. It is unavoidable. But it is easier with build flags if you use VCS: these flags could be different for different versions.
Old code built with a different compiler isn't something that should be just dropped into production.
If you want to do this, you should version the build infrastructure as well as the code.
Otherwise, it's important to look at the new warnings: they might reflect changes in the generated code. Putting -Werror in your default build settings makes sure this happens before something blows up in prod.
Keep master clean of any warnings, for sure. But don’t put it straight into the build system defaults.