[BUG]: MacOS Compilation Error: Name collision between <complex.h> 's I macro and Coords I variables in components ( Union_master , Refractor )

[MilanKlausz]

Mostly AI generated bug report, but it's better than what I would write :)
I ran into issue when using SasView sample component together with Union_master, but the provided simple example reproduces the same error.

Describe the bug
On macOS, compiling instruments that include <tgmath.h> (e.g. when using SasView models or GPU-accelerated configurations) fails with compiler syntax errors when components like Union_master or Refractor are included in the same translation unit.

This occurs because <tgmath.h> automatically includes the C99 <complex.h> header, which defines the imaginary unit macro #define I _Complex_I. This macro collides with the local Coords I variable declarations used in the math sections of Union_master.comp and Refractor.comp.

Document steps to reproduce the bug
Steps to reproduce the behavior:
1. Save the following minimal instrument as test_collision.instr:

  DEFINE INSTRUMENT test_collision()
  
  DECLARE
  %{
  /* 
   * Simulates includes that pull in <complex.h> (like <tgmath.h> for type-generic math).
   */
  #include <tgmath.h>
  %}

  TRACE

  COMPONENT origin = Arm() AT (0,0,0) ABSOLUTE

  COMPONENT ref = Refractor(
    xwidth=0.1, yheight=0.1, zdepth=0.1
  ) AT (0,0,1) RELATIVE origin

  END

2. Compile the instrument using the terminal command:

mcrun -c test_collision.instr

3. See compiler syntax errors pointing to the variable I (which has been macro-expanded to _Complex_I):

test_collision.c:11257:16: error: expected identifier or '('
 11257 |         Coords I = coords_scale (V, 1 / v); // normalised ray = v/|v|
       |                ^
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/complex.h:42:11: note: expanded from macro 'I'
     42 | #define I _Complex_I
         |           ^
  test_collision.c:11260:44: error: passing '_Complex float' to parameter of incompatible type 'Coords'
   11260 |         double cos_theta1 = -coords_sp (N, I); // cos(theta1) = -N.I
         |                                            ^

Expected behavior
The generated C code should compile successfully without syntax errors.

Screenshots
N/A

OS and McStas/McXtrace version (please complete the following information):

• OS: macOS 15.7.7 (Build 24G720), x86_64
• McCode variant: McStas
• Version: 3.7.9

Add your code / dataset
See above.
Command used:

    mcrun -c test_collision.instr

(using Xcode SDK clang as the C compiler).

Additional context
The variable I is declared and used in at least the following places:
1. union/Union_master.comp (around line 2746):
c Coords I = coords_scale (V, 1 / v_length);
2. optics/Refractor.comp (around line 596):
c Coords I = coords_scale (V, 1 / v);

Suggested Fix:
Rename the I variable in the affected components to a less generic name (e.g. I_dir or I_vec representing the normalized ray direction). Using single-letter variables like I is highly prone to collisions with C99 headers.

[willend]

@MilanKlausz thanks for reporting and suggesting.