Operator overloading question

[mist]

Hm, but why can't static assert provide an instantiation trace? I 
can live without error message but duplicating same condition 
twice (one time being part of implementation) hurts my eyes :(

On Monday, 21 January 2013 at 17:16:22 UTC, Ali Çehreli wrote:
> On 01/21/2013 08:32 AM, mist wrote:
>
> > phobos style
> > guidelines favor constraints heavily over static asserts but
> this
> > exactly the example when I am uneasy about such choice:
> static assert
> > can both provide more user-friendly error message here and
> remove some
> > code duplication.
>
> Agreed but there is a problem with 'static assert' in the 
> implementation: We don't know what user code has caused the 
> issue. (It is a common problem in C++ templated libraries that 
> a hard-to-understand compilation error is produced from inside 
> a library function template.)
>
> D's template constraints move the compilation error to the 
> exact place of the user code.
>
> void foo(string s)()
>     if ((s == "hello") || (s == "goodbye"))
> {
>     // ...
> }
>
> void main()
> {
>     foo!"howdy"();  // <-- compilation error on this line
> }
>
> It is better:
>
> Error: template instance foo!("howdy") foo!("howdy") does not 
> match template declaration foo(string s)() if (s == "hello" || 
> s == "goodbye")
>
> To be honest, it is kind of obvious in this simple case but 
> sometimes the cause of the error is still hard to understand 
> even with template consraints.
>
> Ali