Reenable fma

[NAThompson]

No description provided.

[NAThompson]

@mborland, @jzmaddock, @swatanabe : Could your review?

Couldn't figure out how to get this one to compile:

BOOST_TEST(bu::fma(L1, 1.0/L1, 1.0) == 2.0);

but . . . perfect is the enemy of the good.

[mborland]

It seems reasonable to me. I think something of note if it's not already known is this transitively requires C++14 since cmath.hpp includes a number of headers from Boost.Math.

[jzmaddock]

If you wanted to handle all of the mixed argument cases (there are a lot!), then the only sensible way to do it is to provide one fma template that is enable_if'd on at least one of the template parameters being a unit. Then in the body you need to call an "unwrap" helper function that returns the result of argN.value() when argN is a unit, and simply argN otherwise. Hope that all makes sense!

[swatanabe]

LGTM. The other mixed argument cases are not necessary. Multiplication of a quantity by a scalar is allowed. Addition of a quantity and a scalar is not.

[NAThompson]

The other mixed argument cases are not necessary.

These were added due to a compile error I observed on a Horner's algorithm where the abscissa is dimensionless and the coefficients are dimensioned. The compile error when only enabling the original overload is as follows:

In file included from ~/test/polynomial_test.cpp:22:
~/polynomial.hpp:58:11: fatal error: no matching function for call to 'fma'
      y = fma(y, x, c_[i]);
          ^~~
~/.conan2/p/boost21487c6284710/p/include/boost/units/cmath.hpp:242:1: note: candidate template ignored: could not match 'quantity<Unit2, Y>' against 'double'
fma BOOST_PREVENT_MACRO_SUBSTITUTION (const quantity<Unit1,Y>& q1,
^

Code:

  template<typename Real>
  [[nodiscard]] CoefficientType operator()(Real x) const noexcept {
    using std::fma;
    CoefficientType y = c_.back();
    long n = c_.size() - 1;
    for (long i = n - 1; i >= 0; --i) {
      y = fma(y, x, c_[i]);
    }
    return y;
  }

Compiler:

g++ --version
Apple clang version 14.0.3 (clang-1403.0.22.14.1)
Target: arm64-apple-darwin22.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

[NAThompson]

@swatanabe : Mind merging?