The W component should be either positive 0 or 1 depending on what's being represented

point = x,y,z,0
vector = x,y,z,1

Does the issue still occur with vec4(-1,-1,-1,1)?

Yes it also happens when I use vec4(-1,-1,-1,1). I don't think the w component is the issue since the if statement should always be false the code inside should have no effect. It should not be run at all. In GLSL most compilers would remove the code altogether if the statement looks like "if (false)".

I don't think the w component is the issue since the if statement should always be false the code inside should have no effect. It should not be run at all.

The problem is that it will affect the generated glsl code.

I suspect it has something to do with reading the uninited value since adding an else branch will solve the problem :

  bool m_test = false; 
  vec4 position;
  if (m_test) {
    position = vec4(0.0, 0.0, 1.0, 1.0);
  }
  gl_Position = position;

See also the preview window:

This sounds like something to document, I even thought it had already been as it's a pretty standard thing in shader code, but needs documenting in the shader reference section I'd say

Accessing the parameter anywhere makes it dynamic so it is not assigned by the default way, the checks for the compiler are pretty simple, they don't check for dead paths etc., that'd be a lot more work to accomplish

In general for shaders it's important to ensure you don't have undefined paths anyway, because shaders are built to be fast, so they tend to not do safety stuff and ensure initialization etc., to keep it fast

For debugging purposes I'd suggest using the preprocessor instead for this btw:

void vertex() {
#if 0
    POSITION = vec4(...);
#endif
}

Yes, we need to add documentation here to say that if POSITION is written to anywhere in the shader, it will always be used, so the user becomes responsible for ensuring that it always has acceptable values.