[JonChesterfield]

It seems we're in agreement on the mechanism of modules. Thank you for the example - one really large header-only library would seem to be the ideal use case for precompiled headers. Sorry, modules.

There are of course alternatives to header only libraries so there's a sense in which this is a solution to a problem that didn't need to exist in the first place.

But yes, header only libraries are an important solution to the problem of cmake, and modules can patch around the compile time implications of header only libraries, so that's sort of all good.