Using the [[clip_distance]] attribute in a Metal shader?

In a Metal shader, I am trying to make use of the [[clip_distance]] attribute in the output structure of a vertex shader function as follows:

struct vtx_out {

    float4 gl_Position [[position]];

    float gl_ClipDistance[1] [[clip_distance]];

};

However, this results in the following shader compilation error:

<program source>:86:32: error: 'clip_distance' attribute cannot be applied to types

    float gl_ClipDistance[1] [[clip_distance]];

                               ^

I am trying to compile this for running on a Mac running OS X El Capitan.

Why am I getting this error, and how can I make use of the [[clip_distance]] attribute?