Does dmd not always compile all of the source code?

[Atila Neves]

On Wednesday, 6 December 2017 at 19:40:49 UTC, A Guy With a 
Question wrote:
> On Wednesday, 6 December 2017 at 19:19:09 UTC, A Guy With a 
> Question wrote:
>> It seems D's fast compile times are achieved by skipping 
>> semantic checking and even parsing when it doesn't feel it's 
>> needed. I strongly disagree with this decision. This could 
>> leave complex dormant time bombs that break builds 
>> unexpectedly and even accidentally. It's understandable in 
>> certain situations where there is enough information, but the 
>> first step to testing code, is first making sure it 
>> compiles...I don't want the compiler making decisions on what 
>> is worthy to compile. If I pass a d source file into it, I 
>> want to know if it's valid. This is unfortunate. This might be 
>> a deal breaker for me.
>
> I'm very concerned of working with a language that, at minimum, 
> doesn't let me know if a file I passed in even contains valid 
> code.

It does let you know if it contains valid code - if you're 
actually building it.

If you write unit tests but never compile them in, whether or not 
they make any sense is IMHO irrelevant. If you write a template 
and never instantiate it, does it make a sound?*

Imagine this:

version(Windows) int i = 0;
else foobarbaz;

Should it fail to compile on Linux? How is this any different 
from:

#ifdef _WIN32
     int i = 0;
#else
     ohnoes
#endif

As noted by others, C++ templates work similarly. And for good 
reason!

Atila

* https://en.wikipedia.org/wiki/If_a_tree_falls_in_a_forest