function template specialization question D vs. C++

[Adam D. Ruppe]

On Sunday, 14 January 2018 at 00:09:42 UTC, kdevel wrote:
> The compiler does not allow me to specialize the primary 
> function template for double:

Yes, it does., and it works for float and double. Just `real` 
matches both equally, so that's the error for that type. Let me 
quote the spec:

https://dlang.org/spec/template.html#parameters_specialization

"The template picked to instantiate is the one that is most 
specialized that fits the types of the TemplateArgumentList. 
Determining which is more specialized is done in the same way as 
the C++ partial ordering rules. If the result is ambiguous, it is 
an error."


So, if there's only T:float, float, double, and real can all be 
implicitly converted to that, so it is an exact match for float, 
but a half-match for double and real, which are still better than 
unspecialized T, so it matches that.

When you have T:float and T:double, then double, being an exact 
match for the second one, triggers that. Similarly, float is 
still an exact match for the first one, so that's the best. But 
real is a half-match for both - it implicitly converts, but is 
not exactly either. So it is ambiguous - the compiler doesn't 
know which is a best fit.

You can add T:real to exactly match that, or use a constraint to 
filter out real on one or both to remove it from consideration.