[MacPorts] #66646: qt5-qtwebengine: fatal error: 'optional' file not found

[MacPorts]

#66646: qt5-qtwebengine: fatal error: 'optional' file not found
------------------------------+---------------------------------
  Reporter:  mf2k             |      Owner:  MarcusCalhoun-Lopez
      Type:  defect           |     Status:  assigned
  Priority:  Normal           |  Milestone:
 Component:  ports            |    Version:  2.8.99
Resolution:                   |   Keywords:
      Port:  qt5-qtwebengine  |
------------------------------+---------------------------------

Comment (by MarcusCalhoun-Lopez):

 I am afraid I cannot reproduce this error on my machine.\\
 As far as I can tell, [https://build.macports.org/builders/ports-
 13_x86_64-builder/builds/10912 neither can the buildbots].\\
 `optional` is included via `map`, which is perfectly usable with C++14.\\
 I am at a bit of a loss.\\
 Do simple programs like
 {{{
 #include <map>

 int main() {
     return 0
 }
 }}}
 work?

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Ticket URL: <https://trac.macports.org/ticket/66646#comment:5>
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