If you write a shader that uses the texgen planes, you will get complaints about invalid state[0] or something like that.

I haven't cooked up a simple example program to trip this up, but it should not be necessary, because there is obviously a problem with the code.

In src/mesa/shader/slang/slang_builtin.c:
In emit_statevar:

      else if (strcmp(name, "gl_EyePlaneS") == 0) {
         tokens[0] = STATE_TEXGEN_EYE_S;
      }
      ...
      else if (strcmp(name, "gl_ObjectPlaneQ") == 0) {
         tokens[0] = STATE_TEXGEN_OBJECT_Q;
      }

This is inconsistent with _mesa_fetch_state in src/mesa/shader/prog_statevars.c.
My guess (not being familiar with the code) is that it should be

         tokens[0] = STATE_TEXGEN;
         tokens[2] = STATE_TEXGEN_EYE_S;

etc.