Fallback 'catch-all' template functions

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On Thursday, 1 September 2016 at 19:32:23 UTC, Timon Gehr wrote:
>> Well, I'd argue that's not quite right and the correct 
>> interpretation is
>> "If not the other X then this X", due to the 
>> `!__traits(compiles,
>> .f!T)`, explicitly telling the compiler to check if the *other*
>> "overloads" compile.
>
> Even if that was the intention of the compiler implementation, 
> the example below demonstrates why it cannot work.
>
>> I don't actually know whether template constraints
>> are considered to be at module scope or in the template scope, 
>> though,
>> so maybe I'm completely wrong on this.
>> ...
>
> Template scope, but /nothing/ about '.' says "other".

It means to look up the symbol f at module scope, so I guess it 
depends on whether the compiler excludes the current template 
from that lookup.

template f() if (someCondition) {}
template f() if (someOtherCondition) {}
template f() if (!__traits(compiles, { alias _ = .f!(); })) {}

Does `.f` refer to all symbols named f at module scope, or all 
symbols named f *other than* the symbol for which the current 
template constraint is being processed?

Actually, I just convinced myself that it's obviously not the 
latter and so must be the former, and now I see why this 
shouldn't work.