[Issue 18774] meta used in .di files causes link errors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18774

--- Comment #1 from hsteoh at quickfur.ath.cx ---
AIUI, all template functions that are instantiated will end up in the object
file, even if they are only used during CTFE (i.e. the instantiation is
triggered by CTFE but not anywhere else).  This is a problem that's been
bothering me too, which has led me to insert `if (__ctfe)` blocks into my CTFE
functions.  But if you're using Phobos functions, then you're out of luck.

I agree that the compiler should have some way to track whether a particular
template function / instantiation / etc., is only referenced by CTFE code, so
that said functions are omitted from the object file when emitting code. 
Possibly something like a boolean flag in the template symbol to indicate
whether the symbol is referenced by runtime constructs.

I suspect, though, that this is far from trivial to implement, because code
that gets run by CTFE is essentially just one step away from being emitted as
object code, and for all intents and purposes is considered as "runtime" code
by many parts of the compiler.  So you get tricky things like whether a
function called by a CTFE function should be marked as CTFE-only or should be
included in the object file (since that function call would have been resolved
before the CTFE engine even sees it, so it would already have been considered
as "runtime", even if in actuality it will only ever be called from CTFE).

Still, it's a nice-to-have.

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