On Wednesday, 12 May 2021 at 12:54:24 UTC, Per Nordlöw wrote:
I'm certain I've seen dmd be able to diagnose which
sub-expression of `isDumb` and `isInputRange` that evaluated to
false. Did I just dream this or is there a way? If there is a
way does this improvement still reside in a dmd PR?
It will diagnose which sub-expression of the template constraint
failed, but it will not descend into templates. So, if you inline
the constraint:
void f(Args...)(Args args)
if (Args.length == 2 && Args.length == 3)
{
}
You get the following error message:
onlineapp.d(18): Error: template `onlineapp.f` cannot deduce
function from argument types `!()(int, int)`, candidates are:
onlineapp.d(6): `f(Args...)(Args args)`
with `Args = (int, int)`
must satisfy the following constraint:
` Args.length == 3`
It might be possible to extend this to work with simple `enum`
templates like `isDumb` and `isInputRange`, whose bodies consist
of a single boolean expression, although I expect you would run
into many of the same difficulties that caused past attempts at
fixing issue 1807 [1] to fail.
[1] https://issues.dlang.org/show_bug.cgi?id=1807
