Removing the static specifier from a function template

[d7d1cd]

The presence of the static specifier on a template makes it internally linked. This template is used, in particular, to evaluate the NTTP boost::mpl_::bool_, which is needed in boost::multi_index::tag. Internal linkage prevents the use of boost::multi_index::tag and everything that depends on it in the global fragment of C++20 modules.

MRE here https://godbolt.org/z/efM6oaex9

[apolukhin]

@pdimov could you please take a look at this fix and merge if it is OK?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.73%. Comparing base (02ec362) to head (67180b0).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #90   +/-   ##
========================================
  Coverage    94.73%   94.73%           
========================================
  Files            5        5           
  Lines           19       19           
  Branches         3        3           
========================================
  Hits            18       18           
  Misses           1        1           

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