[tor-bugs] #19044 [Core Tor/Tor]: Turn on --enable-gcc-warnings-advisory by default

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#19044: Turn on --enable-gcc-warnings-advisory by default
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     Reporter:  special       |      Owner:
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.9.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Minor         |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 I've had multiple patches fail due to missing the `--enable-gcc-warnings`
 flag in builds. I've also missed really important problems that these
 warnings would tell me about.

 Turning on `--enable-gcc-warnings` by default is probably not a good
 choice, because GCC warnings vary quite a lot between versions and
 `-Werror` will cause these new warnings to break builds.

 Without `-Werror`, there's no harm in enabling all of these warnings by
 default, and this makes us more likely to notice important warnings. We
 should do it.

 I suggest:

 1. Make the behavior of `--enable-gcc-warnings-advisory` default when
 supported by the compiler
 2. Add an `--enable-fatal-warnings` option to turn on `-Werror`
 3. Alias `--enable-gcc-warnings` to `--enable-fatal-warnings` for
 backwards compatibility
 4. Deprecate or ignore `--enable-gcc-warnings-advisory`
 5. Optionally, we could have a `--disable-extra-warnings` flag, if there's
 a case where someone wants to turn off these warnings

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19044>
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