This is an attempt (mostly suggested and implemented by AI, but with a few hours of human babysitting...), to somewhat reduce compilation time by picking one template, named_value<T>, which is used in more than a hundred source files through the config.hh header, and making it use external instantiation: The different methods of named_value<T> for various T are instantiated only once (in config.cc), and the individual translation units don't need to compile them a hundred times.
The resulting saving is a little underwhelming: The total object-file size goes down about 1% (from 346,200 before the patch to 343,488 after the patch), and previous experience shows that this object-file size is proportional to the compilation time, most of which involves code generation. But I haven't been able to measure speedup of the build itself.
1% is not nothing, but not a huge saving either. Though arguably, with 50 more of these patches, we can make the build twice faster :-)
Refs #1.
Closesscylladb/scylladb#28992
* github.com:scylladb/scylladb:
config: move named_value<T> method bodies out-of-line
config: suppress named_value<T> instantiation in every source file
280fe7cfb7