[MacPorts] #58441: "port upgrade libgcc8" pulls libgcc9 instead (and fails, of course). Also affects gcc8 port

[MacPorts]

#58441: "port upgrade libgcc8" pulls libgcc9 instead (and fails, of course). Also
affects gcc8 port
-------------------------+--------------------
  Reporter:  mouse07410  |      Owner:  (none)
      Type:  defect      |     Status:  closed
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:  worksforme  |   Keywords:
      Port:  libgcc8     |
-------------------------+--------------------

Comment (by kencu):

 Maybe I explain better -- we are always open to a "better way" if there is
 one, but this one has been hammered out over some years so I think you
 might find in the end it is quite well thought out.

 I think I have this all correct, but there always be someone with a
 correctional tweak here or there.

 Due to factors of design and compatibility, all versions of gcc that can
 run on a given OS version need to use the **same** libgcc binary. Every
 version of gcc > gcc45 installed on a given OS version uses the same
 libgcc binary, which therefore has to be the latest one the OS version can
 support. Luckily these are always backward compatible, from gcc4.5
 forward, so far.

 Any other apparent libgcc versions installed are simply symlinks to that
 latest libgcc.

 The **latest** libgcc that that the OS version can support is either
 libgcc7, 8, or 9, depending on the OS version.

 So if someone tries to install gcc8 on Mojave, they actually will get
 libgcc9 to support it, and MacPorts will symlink libgcc so that gcc8 is
 happy.

 If you outright tried to force install libgcc8, you will just get libgcc9
 anyway, because otherwise everything will break down. You cannot use
 libgcc8 on newer systems any longer (unless you are Chris or Marcus etc,
 and then if you did figure out how to do that inside MacPorts', things
 would start breaking, so don't do that).

-- 
Ticket URL: <https://trac.macports.org/ticket/58441#comment:9>
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