From d7f6b96e36c3cadb7a2731c6417709ee9668f2d7 Mon Sep 17 00:00:00 2001
From: Nat Goodspeed <nat@lindenlab.com>
Date: Thu, 26 Sep 2024 12:35:27 -0400
Subject: 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.
---
 indra/llmath/llinterp.h         |  4 ++--
 indra/llmath/llmath.h           | 11 ++++++++---
 indra/newview/llcylinder.cpp    |  2 +-
 indra/newview/llsky.cpp         |  2 +-
 indra/newview/llsprite.cpp      |  2 +-
 indra/newview/llworldmipmap.cpp |  2 +-
 6 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/indra/llmath/llinterp.h b/indra/llmath/llinterp.h
index f4faa82a82..386679ffe9 100644
--- a/indra/llmath/llinterp.h
+++ b/indra/llmath/llinterp.h
@@ -29,11 +29,11 @@
 #if defined(LL_WINDOWS)
 // macro definitions for common math constants (e.g. M_PI) are declared under the _USE_MATH_DEFINES
 // on Windows system.
-// So, let's define _USE_MATH_DEFINES before including math.h
+// So, let's define _USE_MATH_DEFINES before including cmath
     #define _USE_MATH_DEFINES
 #endif
 
-#include "math.h"
+#include <cmath>
 
 // Class from which different types of interpolators can be derived
 
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)
 {
diff --git a/indra/newview/llcylinder.cpp b/indra/newview/llcylinder.cpp
index c347d3e1be..d5f1e25151 100644
--- a/indra/newview/llcylinder.cpp
+++ b/indra/newview/llcylinder.cpp
@@ -29,7 +29,7 @@
 #include "llcylinder.h"
 
 #include "llerror.h"
-#include "math.h"
+#include <cmath>
 #include "llmath.h"
 #include "noise.h"
 #include "v3math.h"
diff --git a/indra/newview/llsky.cpp b/indra/newview/llsky.cpp
index 82caa14433..66fbe0029a 100644
--- a/indra/newview/llsky.cpp
+++ b/indra/newview/llsky.cpp
@@ -39,7 +39,7 @@
 // linden library includes
 #include "llerror.h"
 #include "llmath.h"
-#include "math.h"
+#include <cmath>
 #include "v4color.h"
 
 #include "llviewerobjectlist.h"
diff --git a/indra/newview/llsprite.cpp b/indra/newview/llsprite.cpp
index e51aeb6080..09c57eaf1d 100644
--- a/indra/newview/llsprite.cpp
+++ b/indra/newview/llsprite.cpp
@@ -37,7 +37,7 @@
 
 #include "llsprite.h"
 
-#include "math.h"
+#include <cmath>
 
 #include "lldrawable.h"
 #include "llface.h"
diff --git a/indra/newview/llworldmipmap.cpp b/indra/newview/llworldmipmap.cpp
index d8ea2b884f..d198f7a33b 100644
--- a/indra/newview/llworldmipmap.cpp
+++ b/indra/newview/llworldmipmap.cpp
@@ -30,7 +30,7 @@
 #include "llviewercontrol.h"        // LLControlGroup
 
 #include "llviewertexturelist.h"
-#include "math.h"   // log()
+#include <cmath>   // log()
 
 // Turn this on to output tile stats in the standard output
 #define DEBUG_TILES_STAT 0
-- 
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