overlap_check: make the ray direction independent of the compiler

doc/CHANGELOG.rst

@@ -21,6 +21,8 @@ Next version
     steps stop the job instead of masking the real error (#994)
   * Fixed the changelog check, which had been failing on checkout since the Node 20
     deprecation, by dropping the alpine:3.14 container (#994)
+  * Made the overlap_check ray direction independent of the compiler, which had
+    been picking a different direction under GCC than under Clang (#996)
 
 
 v3.2.4

src/overlap_check/overlap.cpp

@@ -91,7 +91,15 @@ ErrorCode check_instance_for_overlaps(std::shared_ptr<Interface> MBI,
   int num_locations = all_verts.size() + pnts_per_edge * all_edges.size();
   int num_checked = 1;
 
-  CartVect dir(rand(), rand(), rand());
+  // The order in which function arguments are evaluated is unspecified, so
+  // calling rand() three times inside the constructor picks a different
+  // direction depending on the compiler: GCC evaluates right to left, Clang
+  // left to right. Evaluate the calls in a defined order instead, so that the
+  // same geometry gives the same answer whichever compiler was used.
+  const double dir_x = rand();
+  const double dir_y = rand();
+  const double dir_z = rand();
+  CartVect dir(dir_x, dir_y, dir_z);
   dir.normalize();
 
   ProgressBar prog_bar;