Reinstate our lerp() function, avoid "math.h" header.

For reasons that remain unclear, MSVC likes our lerp() function better than
its own std::lerp() function: publishing the latter into the global namespace,
instead of defining our own, produces fatal argument conversion warnings.

"math.h" publishes all of <cmath> into the global namespace, which causes a
GCC conflict between std::lerp() and our lerp() function. Including <cmath>
instead leaves std::lerp() in the std namespace, eliminating the conflict.

1 files changed, 8 insertions, 3 deletions

diff --git a/indra/llmath/llmath.h b/indra/llmath/llmath.h
index 09b685a68b..75284ef57e 100644
--- a/indra/llmath/llmath.h
+++ b/indra/llmath/llmath.h
@@ -358,9 +358,14 @@ inline F32 snap_to_sig_figs(F32 foo, S32 sig_figs)
     return new_foo;
 }
 
-// We used to define a simple lerp(F32, F32, F32) that probably predated
-// std::lerp(). By now, do we need our own definition any longer?
-using std::lerp;
+// Even though there's now a std::lerp() function that appears to do the same
+// as this function, for some reason MSVC likes this one better. Publishing
+// std::lerp() into the global namespace instead of defining this function
+// results in fatal argument conversion warnings.
+inline F32 lerp(F32 a, F32 b, F32 u)
+{
+    return a + ((b - a) * u);
+}
 
 inline F32 lerp2d(F32 x00, F32 x01, F32 x10, F32 x11, F32 u, F32 v)
 {