GLSL spec says: "User code can replace built-in functions with their own if they choose, by simply re-declaring and defining the same name and argument list."

But piglit/tests/glslparsertest/shaders/CorrectBuiltInOveride.frag fails when defining 'radians' (a built-in function) with below failure: 
error: function `radians' redefined

This happens with mesa glsl2 branch, while passes with master branch.

It seems the glsl2 ir reader doesn't consider built-in functions when processing redefining error.