January 30, 2006
The behaviour of static assert() has changed again with DMD 0.145, and it tends not to trigger error messages any more, as in the example below. I included a pragma(msg) to show that the static assert is actually evaluated, but ignored. Variants of this can crash the compiler, but I haven't cut it down to a simple case yet.
--------
template echidna(int z)
{
  pragma(msg, "This actually gets evaluated!");
  const bool quoll = false;
}

template dingo(int a)
{
  static if (a==2) {
    static assert(echidna!(a).quoll);
  } else const int dingo = a;
}
const int bilby = dingo!(2);
-----
An annoying aspect of this behaviour is that you are not given the line which asserted.

But rather than simply fix some superficial bugs, there's a potential for much better behaviour.

(1) It would be *much* nicer if static assert and pragma() did not
interfere with the implicit promotion of template members.
Both static assert() and pragma(msg, ...) are very useful in tracking down template bugs, but since they alter the effect of the program,
they're extremely awkward to use right now (and it seems conceptually wrong).

(2) Ideally, when a failed static assert() occurred, the compiler would simply show the context (follow the syntax tree back down to the root, printing the line being evaluated, but without any new error messages).
This might be as simple as setting a global "supressParasiticErrorMessages" boolean. (?) There's really no point in trying to continue compiling once you've hit a static assert. Too much of the symbol table is erroneous. (In DMD 0.144, once a static assert failed, _every_ template instantiation from then on would fail! Not sure if 0.145 is the same).

Together, these two things would allow D template libraries to make highly user-friendly error messages (in marked contrast to the STL!)
January 31, 2006
Don Clugston wrote:
> The behaviour of static assert() has changed again with DMD 0.145, and it tends not to trigger error messages any more, as in the example below. I included a pragma(msg) to show that the static assert is actually evaluated, but ignored. Variants of this can crash the compiler, but I haven't cut it down to a simple case yet.
> --------
> template echidna(int z)
> {
>   pragma(msg, "This actually gets evaluated!");
>   const bool quoll = false;
> }
> 
> template dingo(int a)
> {
>   static if (a==2) {
>     static assert(echidna!(a).quoll);
>   } else const int dingo = a;
> }
> const int bilby = dingo!(2);
> -----
> An annoying aspect of this behaviour is that you are not given the line which asserted.
> 
> But rather than simply fix some superficial bugs, there's a potential for much better behaviour.
> 
> (1) It would be *much* nicer if static assert and pragma() did not
> interfere with the implicit promotion of template members.
> Both static assert() and pragma(msg, ...) are very useful in tracking down template bugs, but since they alter the effect of the program,
> they're extremely awkward to use right now (and it seems conceptually wrong).
> 
> (2) Ideally, when a failed static assert() occurred, the compiler would simply show the context (follow the syntax tree back down to the root, printing the line being evaluated, but without any new error messages).
> This might be as simple as setting a global "supressParasiticErrorMessages" boolean. (?) There's really no point in trying to continue compiling once you've hit a static assert. Too much of the symbol table is erroneous. (In DMD 0.144, once a static assert failed, _every_ template instantiation from then on would fail! Not sure if 0.145 is the same).
> 
> Together, these two things would allow D template libraries to make highly user-friendly error messages (in marked contrast to the STL!)

I agree with Don, when static assert fails, the compile should stop, then show the line of the assert which failed.
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