I got this error:
Compile failed.
ERROR: 0:163: 'smoothstep' : cannot redeclare built-in function (only overloading is allowed)
The GLSL shader explicitely started with this line:
As far as I know, smoothstep isn't part of GLSL 1.20.
Our code declares some functions like smoothstep for GLSL versions not implementing them.
So our code then expects smoothstep to not be declared when explicitely requesting GLSL 1.20.