* include/std/numeric (midpoint(T8, T*)): Do not check for complete
type during overload resolution, use static assert instead (LWG 3200).
* testsuite/26_numerics/midpoint/pointer.cc: Do not test with
incomplete type.
* testsuite/26_numerics/midpoint/pointer_neg.cc: New test.
The implementation of midpoint used for integral types is due to Howard
Hinnant and avoids a branch for int and larger types (but not for chars
and shorts).
The midpoint and lerp functions for floating point types come straight
from the P0811R3 proposal, with no attempt at optimization.
* include/c_compatibility/math.h [C++20] (lerp): Add using
declaration.
* include/c_global/cmath [C++20] (__cpp_lib_interpolate): Define.
(__lerp): Define function template to implement lerp.
(lerp(float, float, float), lerp(double, double, double))
(lerp(long double, long double, long double)): Define for C++20.
* include/std/numeric [C++20] (__cpp_lib_interpolate): Define.
(midpoint(T, T), midpoint(T*, T*)): Define.
* include/std::version [C++20] (__cpp_lib_interpolate): Define.
* testsuite/26_numerics/lerp.cc: New test.
* testsuite/26_numerics/midpoint/floating.cc: New test.
* testsuite/26_numerics/midpoint/integral.cc: New test.
* testsuite/26_numerics/midpoint/pointer.cc: New test.
From-SVN: r269398
