Comment # 12 on bug 66067 from Nicholas Miell

(In reply to comment #11)
> > ARB_fragment_program_shadow may leave it undefined, but the GL spec ...
> 
> Which spec exactly? GL specifications only cover GLSL shaders.
> ARB_fragment_program is separate from that and has its own rules. The
> extension specification clearly states that behaviour is undefined.
> 
Section 8.23.1 of the GL 4.4 spec, which I paraphrased in comment #9.