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The hard to detect is the compilers technical reason for not doing the analysis. Maybe it will one day.

In practice, it's annoying to track if those small util functions become dead code.

i did not say "Impossible to detect"!

in practice the exponential explosion of options may become intractable. lets say you have 10 compilation flags with 10 options each. not syre you want the compiler scanning through all that on each pass

Yes - I was only talking about when a person removes some code from use, removing the code as well as the call to the code. I wasn't meaning the compiler at all (although it would be nice if the compiler could autodetect it, as you say, that could become tricky).

That requires a person to be making that change. If the change is vibe coded and the person is only checking that the tests pass, it's not hard for cruft to accumulate.