Fixed lerp() duplicate when compiling for C++20

[niansa]

Hey!

I've noticed luau failing to compile on Clang:

luaaau/luau/VM/src/lmathlib.cpp:303: error: declaration conflicts with target of using declaration already in scope
luaaau/luau/VM/src/lmathlib.cpp:303:14: error: declaration conflicts with target of using declaration already in scope
static float lerp(float t, float a, float b)
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/cmath:1911:3: note: target of using declaration
  lerp(float __a, float __b, float __t) noexcept
  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/math.h:183:12: note: using declaration
using std::lerp;
           ^

So I looked at the math.h file and found this:

#if __cplusplus > 201703L
using std::lerp;
#endif // C++20

So it seems like std::lerp() is put into the global namespace above C++17. That means that we already have a lerp() implementation in the global namespace! I decided to simply do this in luaus code:

#if __cplusplus <= 201703L
static float lerp(float t, float a, float b)
{
    return a + t * (b - a);
}
#endif

And voila, everything is compiling just fine now.

This probably isn't the cleanest solution, and I initially thought it simply was a Clang issue when I commited. Please comment your thoughts. 😄

Niansa

[niansa]

I wasn't able to test this on MSVC, just GCC and Clang. Does the CI run tests with C++20 selected?

[vegorov-rbx]

lerp and std::lerp actually have slightly different performance and precision 🙄
It would be better to rename lerp to math_lerp like we do to other functions in the file.

[niansa]

Here we go :-)

[vegorov-rbx]

Thank you!