I'm running Windows 10 64-bit, latest drivers, R9 390.
If I include "#extension GL_AMD_vertex_shader_viewport_index : require" within a vertex shader I get:
ERROR: 0:5: error(#61) Required extension is not found. Extension "GL_AMD_vertex_shader_viewport_index" is not supported
ERROR: error(#273) 1 compilation errors. No code generated
This is despite the fact that querying the extension reports it as supported, either via "glewIsExtensionSupported" or "glGetString(GL_EXTENSIONS)". I've tried a combination of profile versions, GLSL versions, etc, all the same result. Note this works 100% fine on NVIDIA cards. Am I doing something wrong, or is this a driver bug?
Simple test-case attached, note the console output: http://downloads.flyinside-fsx.com/AMDExtensionBug.zip