"temporary" templates

[Paul Backus]

On Tuesday, 26 November 2019 at 18:45:35 UTC, Adam D. Ruppe wrote:
> On Tuesday, 26 November 2019 at 18:35:42 UTC, Stefan Koch wrote:
>> In _general_ it's not possible to categorize a template  as 
>> being temporary or  not.
>> For language semantic reasons it is a requirement  for every 
>> re-instantiated template to forward  to  exactly the same 
>> symbol as was generated during the first instantiation.
>
> What if it kept *just* the symbol, but discarded its innards 
> when under memory pressure, then could regenerate them if 
> evaluated again?

This will cause compilation to become non-deterministic, since 
the result of template evaluation can depend on the order in 
which declarations are semantically analyzed:

enum hasFoo(T) = __traits(hasMember, T, "foo");

struct S {
     // hasFoo!S is instantiated before the mixin is processed
     static if (hasFoo!S) {}
     mixin("int foo;");
}

pragma(msg, hasFoo!S); // false

If for some reason the "innards" of `hasFoo!S` are discarded due 
to memory pressure, reconstructing them later will give a 
different result.