What you’re seeing is actually normal for bgfx’s shaderc: your .sc source isn’t fed directly to GLSL or SPIR-V. Instead, shaderc first runs it through bgfx’s shader preprocessor, which expands $input, $output, $varyingdef, etc. into HLSL-style code. That’s why the error points to lines with things like vec4_splat(0.0) — they were inserted by the bgfx shader system, not written by you.

ERRORRR.... 'vec4_splat' no matching overloaded function found

means that shaderc’s generated code tried to call vec4_splat, but that helper wasn’t defined in the scope of your shader. This happens when you don’t include the bgfx shader include files that define all the helper functions (vec4_splat, packing/unpacking, etc.).