std::abs() not considered constexpr by clang

[holynski]

When compiling a fresh clone on OSX 10.14.3:

src/theia/sfm/estimators/estimate_radial_distortion_homography_test.cc:73:25: error: constexpr variable 'kRadDistThreshold1' must be initialized by a constant
expression
static constexpr double kRadDistThreshold1 =
^
src/theia/sfm/estimators/estimate_radial_distortion_homography_test.cc:74:11: note: non-constexpr function 'abs' cannot be used in a constant expression
0.1 * std::abs(kRadialDistortion1 * std::pow(kFocalLength1, 2));
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/math.h:737:1: note: declared here
abs(double __lcpp_x) _NOEXCEPT {return ::fabs(__lcpp_x);}
^

Seems like Clang++ (v10.0) on OSX doesn't allow std::abs to be used in the definition of a constexpr. The same is true for std::pow().

I have a fix for this (see below), and have tried making a PR - but it looks like when I clone from GerritHub, it gives me a significantly older version of the repo (with most recent commit 14421f7), and this file didn't exist back then. I'm following the instructions on here, and cloning from this repo: https://review.gerrithub.io/sweeneychris/TheiaSfM. Did I make a mistake somewhere?

Original code:

// allowing 10 percent error for radial distortion parameters
static constexpr double kRadDistThreshold1 =
    0.1 * std::abs(kRadialDistortion1 * std::pow(kFocalLength1,2));
static constexpr double kRadDistThreshold2 =
    0.1 * std::abs(kRadialDistortion2 * std::pow(kFocalLength2,2));

The fix:

namespace {

  constexpr double ConstexprAbs(const double in) {
    return in >= 0 ? in : -in;
  }

}  // namespace

// allowing 10 percent error for radial distortion parameters
static constexpr double kRadDistThreshold1 =
    0.1 * ConstexprAbs(kRadialDistortion1 * kFocalLength1 * kFocalLength1);
static constexpr double kRadDistThreshold2 =
    0.1 * ConstexprAbs(kRadialDistortion2 * kFocalLength2 * kFocalLength2);

[holynski]

For reference:
https://stackoverflow.com/questions/27708629/why-isnt-abs-constexpr