Josh,

Welcome!

Josh Marshall 写道：
> The problem which we can't figure out is why the compiler being 
> used
> is the default for `gcc` at 7.4 while there is explicit use and
> dependency on `gcc` 9.2 via `gcc-9`.  We're stumped.

Well…  I'm easily stumped but that's actually not the case here.

You've added gcc-9 to ‘inputs’.  If you rename your ‘inputs’ field 
to ‘native-inputs’ you'll the see the magic kick in:

  $ guix build -f magic_enum.scm
  […]
  g++ (GCC) 9.2.0
  Copyright (C) 2019 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions. 
  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A 
  PARTICULAR PURPOSE.

:-)

After removing the debugging ‘ --version’, it complains that:

  cc1plus: error: command line option ?-std=gnu17? is valid for 
  C/ObjC but not for C++ [-Werror]

True dat.  So I edit your code to read:

  #:configure-flags '("-DCMAKE_CXX_FLAGS=-std=c++17")

Note that there's no need for 'quotes' here.  Guix doesn't pass 
command lines to a shell like many other build systems.

Et pouf!

  $ guix build -f magic_enum.scm
  […]
  /gnu/store/h9npwdihnwr11xdbqjr20q5w2r6dbw1x-magic-enum-0.6.3

  $ tree 
  /gnu/store/h9npwdihnwr11xdbqjr20q5w2r6dbw1x-magic-enum-0.6.3
  /gnu/store/h9npwdihnwr11xdbqjr20q5w2r6dbw1x-magic-enum-0.6.3
  ├── include
  │   └── magic_enum.hpp
  ├── lib
  │   └── cmake
  │       └── magic_enum
  │           ├── magic_enumConfig.cmake
  │           └── magic_enumConfigVersion.cmake
  └── share
      └── doc
          └── magic-enum-0.6.3
              └── LICENSE

Glorious.

> Resending because I didn't get a post acknowledgement.

I don't think (first) posts are explicitly acknowledged.  The 
mailing lists are sometimes delayed (seldom more than a few 
minutes).  The Web archives have a similar but often longer delay.

> nckd and I have gone back and forth over a few days in IRC

Close ;-)  I know, weird nick, see my real name for an 
explanation.

Kind regards,

T G-R