Guard redundant constexpr static member redeclarations for C++17+

Pavel Guzenfeldrequested to merge
PavelGuzenfeld/eigen:fix-deprecated-constexpr-redecl into master

Summary

Wrap out-of-class constexpr static data member definitions in numeric_limits_half_impl (Half.h) and numeric_limits_bfloat16_impl (BFloat16.h) with #if EIGEN_COMP_CXXVER < 17 guards.

Since C++17, constexpr static data members are implicitly inline, making these definitions redundant and deprecated. This eliminates 46 -Wdeprecated warnings when building with C++17 or later.

Testing

  • GCC 15.2.0 with -std=c++20 -Wdeprecated: 0 warnings (was 46)
  • GCC 15.2.0 with -std=c++14 -Wdeprecated: 0 warnings (unchanged)
  • half_float test: pass
  • bfloat16_float test: pass
  • cxx11_tensor_scan test: pass

Closes #3061 (closed)

Related to #3015 (closed)

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