From @bmitov on November 4, 2015 1:36

I am testing this simple code with a namespace, and I am getting "sketch_nov03a:8: error: 'namespaceTest' does not name a type" error.
This works perfectly fine in any older version of the IDE . How to enable Arduino IDE 1.6.6 to compile namespaces? I need them for my project.

namespace Test
{
}

void setup() {
// put your setup code here, to run once:
}

void loop() {
// put your main code here, to run repeatedly:
}

Copied from original issue: arduino/Arduino#4075