What is the correct behavior for default_target_compile when type is "none"?

I have been working on writing tests for default_target_compile, 
initially with a goal to rewrite default_target_compile, but now I have 
added another goal:  documenting default_target_compile, since the 
existing entry in the manual is a stub.

I have gathered lists of recognized types and options from the code and 
am now perplexed at what type "none" is actually supposed to do -- the 
current implementation ends up invoking the compiler with no specified 
mode or output, which produces a.out as an executable here.

I understand allowing no output file with "preprocess" -- GCC emits the 
preprocessed output on stdout, which becomes the return value of 
default_target_compile.  But what is type "none" supposed to do?

-- Jacob