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Fix "-Wundef" warning when building as C++20 #85

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Commits on May 6, 2024

  1. Fix "-Wundef" warning when building as C++20

    When including `span.hpp` in a source file compiled as C++20, I get an
    error similar to this:
    
        $ g++ -x c++-header span.hpp -Wundef -Werror -std=c++20
        span.hpp:43:33: error: "span_HAVE_STRUCT_BINDING" is not defined, evaluates to 0 [-Werror=undef]
           43 | #define span_HAVE( feature )  ( span_HAVE_##feature )
              |                                 ^~~~~~~~~~
        span.hpp:1873:5: note: in expansion of macro ‘span_HAVE’
         1873 | #if span_HAVE( STRUCT_BINDING )
              |     ^~~~~~~~~
    
    Fix it by checking if `span_HAVE_STRUCT_BINDING` is defined.
    Unfortunately, I couldn't find a way to make `defined()` work with
    `span_HAVE( STRUCT_BINDING )`.
    
    An alternative would be to move the definition of
    `span_HAVE_STRUCT_BINDING` outside of the `span_USES_STD_SPAN` ifdef.
    
    Fixes martinmoene#84.
    
    Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
    simark committed May 6, 2024
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