FragColor variable name is implicitly reserved on GL3Plus

[altren]

Simple fragment glsl shader produce error, that was not trivial to debug:

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ERROR: 0:22: Regular non-array variable 'FragColor' may not be redeclared

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#version 130
precision highp int;
precision highp float;

uniform sampler2D sampleTexture;
in vec4 outUV0;
in vec4 outColor;
out vec4 FragColor;

// Texturing fragment program for GLSL
void main()
{
	FragColor = outColor * texture(sampleTexture, outUV0.xy);
}

It took me a while to figure out, that FragColor is defined inside Ogre's code in RenderSystems/GL3Plus/src/GLSL/src/OgreGLSLShader.cpp :

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        if(shouldUpgradeToVersion150)
        {
            if(belowVersionPos != 0)
                mSource = mSource.erase(0, belowVersionPos); // drop old definition

            // automatically upgrade to glsl150. thank you apple.
            const char* prefixFp =
                    "#version 150\n"
                    "#define varying in\n"
                    "#define texture1D texture\n"
                    "#define texture2D texture\n"
                    "#define texture3D texture\n"
                    "#define textureCube texture\n"
                    "#define texture2DLod textureLod\n"
                    "#define texture2DProj textureProj\n"
                    "#define textureCubeLod textureLod\n"
                    "#define shadow2DProj textureProj\n"
                    "#define gl_FragColor FragColor\n"
                    "out vec4 FragColor;\n";

I suggest to rename FragColor to something that would not clash with user's variables (for example Ogre_FragColor)
I can create pull request if suggested change is ok.

[paroj]

thats a known issue: https://github.com/OGRECave/ogre/issues/841

renaming FragColor wont get you far. Actually I think hitting that name conflict helped you figuring the issue.

As described in the issue the proper fix would be checking whether gl_FragColor is found in the source code. Feel free to make a PR for that.

Or bump your version to #version 150

[altren]

Simply renaming FragColor to FragColor1 in my shader code solved the issue. I guess that this solution is not perfect, but it works for me.
I agree that this is incomplete solution, though