2018-12-03  Edward Smith-Rowland  <3dw4rd@verizon.net>

	PR libstdc++/88341 - Complex norm doesn't compile with C++11
	* include/std/complex (_S_do_it): Make C++20 constexpr.
	* testsuite/26_numerics/complex/value_operations/pr88341.cc: New test.