[Bug 92743] Centroid shouldn't have to match between the FS and the VS

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92743

--- Comment #5 from Corentin Wallez <corentin at wallez.net> ---
Here is a copy-paste of a bug I filed against dEQP that explains the situation.
I'm not sure what the right step forward is yet, we might have to file a
Khronos bug to have a resolution. In the meantime the centroid-only patch seems
safe to merge, but not the other one.

The GLES3.functional.shaders.linkage.varying.rules.differing_interpolation_2
test tests that these two varyings correctly link together.
 - smooth out mediump float var;
 - centroid in mediump float var;
The differences between the variables is two-fold, no-centroid vs. centroid and
smooth interpolation vs. default interpolation. The spec says that centroid and
no-centroid is ok, see longer explanation in the first comment of
https://bugs.freedesktop.org/show_bug.cgi?id=92743
However the spec unclear on the smooth vs. default being allowed. The GLSL ES
3.0.4 and GLSL 330 spec say in the interpolation section that:
    The type and presence of the interpolation qualifiers and storage
qualifiers and invariant qualifiers of variables with the same name declared in
all linked shaders must match, otherwise the link command will fail. (end of
section 4.3.9 of the GLSL ES 3.0.4 spec)
Which seems to indicate that default vs. smooth is not valid since default is
the absence of qualifier.

However in bug https://cvs.khronos.org/bugzilla/show_bug.cgi?id=7819 a block of
text was added to the spec starting from GLSL 430 stating that the
interpolation qualifiers do not have to link, and that the fragment shader's
one is the one that gets used. So starting from that point on, the spec
contradicts itself.

Now the question is whether the edit to GLSL 430 spec shows what the intent was
all along, in which case we should ignore all the previous spec text on the
presence having to match. Or it is something that should start to take effect
only with the GLSL 430 spec onward in which case there is a spec bug for these
versions.

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