On Saturday, 14 February 2015 at 17:00:33 UTC, Andrei
Alexandrescu wrote:
There's been recurring discussion about failing constraints not
generating nice error messages.
void fun(T)(T x) if (complicated_condition) { ... }
struct Type(T)(T x) if (complicated_condition) { ... }
If complicated_condition is not met, the symbol simply
disappears and the compiler error message just lists is as a
possible, but not viable, candidate.
I think one simple step toward improving things is pushing the
condition in a static_assert inside type definitions:
void fun(T)(T x) if (complicated_condition) { ... } // no change
struct Type(T)(T x)
{
static assert(complicated_condition, "Informative message.");
...
}
This should improve error messages for types (only). The
rationale is that it's okay for types to refuse compilation
because types, unlike functions, don't overload. The major
reason for template constraints in functions is allowing for
good overloading.
Andrei
This seems like a lot of manual labor to cover a compiler
limitation that is going to affect not just Phobos, but every D
project which uses constraints (or template type specialization?)
instead of interfaces. Ideally, the compiler would be able to
say, "Well the function name and the parameter list exactly match
these possibilities, so let's run through each constraint applied
in the set of possibilities and explicitly dump the boolean
result for each constraint."
