[CMake] digging in the dark corners of the command line argument interpreter

[Axel Roebel]

Dear CMakers

I run into a little problem with the command line flag -D.
I'll often use the command line flag -D to pass variables to cmake.
Because typing long names like

-DTT_EXECUTABLE:PATH=tt

is boring I was pretty happy to see that I could do
 
-DTT_EXECUTABLE=tt

as well. Before you start shortcutting I have to add
some salt to this nice soup because unfortunately 
it turns out that this nearly always works besides
if in CMakeLists.txt I do a FIND_PROGRAM.  For example with

MESSAGE("varstate: ${TT_EXECUTABLE}")
FIND_PROGRAM(TT_EXECUTABLE NAMES whatever)
MESSAGE("varstate: ${TT_EXECUTABLE}")

what happens is

$> cd /home/roebel/tmp/
$> cmake . -DTT_EXECUTABLE=whatever
varstate: whatever
varstate: /home/roebel/tmp/whatever

which shows that FIND_PROGRAM replaced the content of variable TT_EXECUTABLE
by $PWD/${TT_EXECUTABLE} which is nonsense most of the time. 
After some more digging I found that with 
-DTT_EXECUTABLE=whatever no cache variable is created 
so the variable seems to be in kind of a zomby state.
However, it turns out that

cmake . -DTT_EXECUTABLE:=whatever

works correctly creating a string type cache variable. 

So the questions are, 
1. what is 
-DTT_EXECUTABLE=whatever
supposed to do in contrast to

-DTT_EXECUTABLE:=whatever
or the book version
-DTT_EXECUTABLE:STRING=whatever

2.
can I rely on -DTT_EXECUTABLE:=whatever to work or am I still abusing a 
current weakness of the command line type check.

Thanks for any illumination!

-- 
Axel Roebel
IRCAM Analysis/Synthesis Team
Phone: ++33-1-4478 4845 | Fax: ++33-1-4478 1540