Template constraints error messages using cputs()

[Andrei Alexandrescu]

bearophile wrote:
> In D1 there were no template constraints, so to test the template arguments I used to add some static asserts inside them. So a wrong template argument shows a error message written by me that explains why the instantiation has failed (unfortunately those error messages show the line number inside the template).
> 
> When a template constraint is composed of some different parts in &&, it's less easy to understand what condition has failed, so I miss the error messages written by me. This is an example (that probably I will simplify), there are four conditions, and for a person that has not written this code, and is just using Phobos, it's not immediately obvious what part has failed:
> 
> 
> auto opBinary(string op, TOther)(TOther other)
>   if (op == "~" && is(TOther == struct) &&
>       (!__traits(compiles, { void isTuple(U...)(Tuple!U){} isTuple(other); }) ||
>        distinctFieldNames!(T, TOther.TypesAndStrings)() )) { ...
> 
> 
> There is a simple solution. The simple template constraints often don't need extra error messages, so they can be left as they are now.
> 
> Putting one or more error message inside a template constraints turns them into messy code, so in such cases it's better to move the tests elsewhere, in an external template/CTFE:
> 
> template IsGoodFoo(T) {
>     static if (...) {
>         enum bool IsGoodFoo = true;
>     } else {
>         ctputs("this is an error message");
>         return false;
>     }
> }
> 
> void foo(T)(T x) if (IsGoodFoo!T) { ...
> 
> 
> So the ctputs() I have suggested here is usable for the template constraints error messages too (this 3952 is in truth a bug report and enhancement request fused in one, later Brad has explained me to avoid this):
> http://d.puremagic.com/issues/show_bug.cgi?id=3952
> 
> This shows one more purpose for ctputs().
> Later I will add those comments to the bug 3952.
> 
> Bye,
> bearophile

pragma(msg, "Is this what you need?");

Andrei