title: Calling `std.math.clamp` gives compile error `error: unable to evaluate constant expression` in Zig
body: <p>Why does this program fail to compile? Zig version 0.6.0.</p>
  <pre><code>const std = @import(&quot;std&quot;);
  fn get_value () f32 {
      return 1.0;
  }
  test &quot;testcase&quot; {
      const value: f32 = 1. + get_value() ;
      _ = std.math.clamp(value, 0.0, 255.0);
  }
  </code></pre>
  <p>Gives compile error:</p>
  <pre><code>$ zig test src/clamp.zig       
  Semantic Analysis [790/1017] ./src/clamp.zig:9:24: error: unable to evaluate constant expression
      _ = std.math.clamp(value, 0.0, 255.0);
                         ^
  ./src/clamp.zig:9:23: note: referenced here
      _ = std.math.clamp(value, 0.0, 255.0);
                        ^
  </code></pre>