Strange. Mine doesn't have an error for that:

I'm doing the same thing. One gdshaderinc with the define, another gdshaderinc with the ClampCosine that includes the first file, then a gdshader file that includes the second file.

Which version of Godot are you using? Shader macros was a recent addition, its new enough that the 4.0 docs say it doesn't exist (they explicitly state GLSL macros like #define can't be easily ported to Godot). It's working for me in 4.0 beta 3 and 4.0 alpha 15, but it might be broken earlier than that.

Another idea: what does the end of definitions.gdshaderinc look like? Maybe there's a stray character or something that is coming in with the include and affecting the ClampCosine (since it's the first thing after the include that isn't a comment).

    Kojack That's odd! Yes I'm on beta 3 as well. There isn't anything weird on the definitions,
    Maybe I should start the project again, just to be sure.

      Kojack No, a completely new project with just the lines in question, still gives me the same error

        Kojack Did you do anything else apart from defining those shaders?
        You project is working, but mine, with the exact two files was not working yesterday.

          I added a plane mesh to a new project, gave it a shader material, gave that a default shader, then copy pasted that file to make the two include files.

          What kind of shader are you making? There's spatial (I used that), canvas and particle. Maybe only spatial is working and canvas or particle aren't complete yet.

            Kojack I figured something out at least, In my .gdshaderinc files I want to use PI, but it does not exist, so I had to define it inside the definitions.gdshaderinc as such:

            const float PI = 3.14159265358979323846f;

            But it seems that PI is already defined in a .gdshader file so in your project I get

            And oddly in mine (which is the exact same as yours only that I started it on my own from scratch) I get the same error as before inside functions.gdshaderinc.

            btw: the error message is about line 121 because I copy pasted all the contents of my original definitions and functions in your respective shader files.

            btw2: another, possibly unrelated issue, in your project if I decide to use an assert

            Number ClampCosine(Number mu) {
  assert(1 <= 2);
  return clamp(mu, Number(-1.0), Number(1.0));
}

            Then I get the following error:

            I don't know if all these are bugs or what!

              5 days later

              greenleafone7 I think I found a fix for this. Are your shaders located in a folder? If so try giving the full path to the include
              #include "definitions.gdshaderinc" --> #include "YourShaderFolder/definitions.gdshaderinc"
              I found this while I was also trying to port the Atmospheric scattering stuff into Godot

              As for the error with assert, it's because glsl and gdshader doesn't have an assert function.
              In Bruneton's atmosphere implementation, he actually combines the shader files in code (model.cc) and adds this to the top

              #define IN(x) const in x
#define OUT(x) out x
#define assert(x)

              This removes all the asserts and also changes the INs and OUTs into glsl versions.

                  5 days later

                  GreatOdds Are you also trying to port this in Godot. That's awesome. I'd definitely like to have a look at your work, or have someone to ask haha. This is a great learning exercise, after porting all the Shadertoy glitch shaders to godot.

                  Right now I am stuck at the use of TEMPLATE which is something I haven't seen before.

                      greenleafone7 The one I'm porting is this. I think your one is also based on this. They also have a full documentation here which helps a lot.

                      TEMPLATE is also another function that gets removed by the preprocessor. Check out model.cc in the documentation link. In the line glsl_header_factory_ you can see the stuff being defined and removed by the preprocessor.

                      I'm not sure how well this will perform in gdshaders. You're going to have to use viewports and stuff to get a output from these. That's going to be especially difficult with the sampler3d for the scattering textures.

                      What I'm doing right now is implementing it in the new compute shaders. This let's me control when the shaders run instead of them running all the time like gdshaders. I can also store the output for later but It's a bit hard without any documentation.

                          GreatOdds That sounds interesting for sure. I need to try this myself, even though I'm a beginner in shaders.