• Per =?UTF-8?B?Tm9yZGzDtnc=?= (37/37) May 12 2021 Given
• Paul Backus (21/25) May 12 2021 It will diagnose which sub-expression of the template constraint

Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:

Given

```d
import std.range.primitives : isInputRange;

enum isDumb(Args...) = (Args.length == 2 && Args.length == 3);

void f(Args...)(Args args)
if (isDumb!(Args))
{
}

void g(Arg)(Arg arg)
if (isInputRange!(Arg))
{
}

 safe pure unittest
{
     f(2, 3);
     g(2);
}
```

`dmd` diagnoses the first level as

```
/home/per/f.d(18,6): Error: template `f.f` cannot deduce function 
from argument types `!()(int, int)`, candidates are:
/home/per/f.d(6,6):        `f(Args...)(Args args)`
   with `Args = (int, int)`
   must satisfy the following constraint:
`       isDumb!Args`
/home/per/f.d(19,6): Error: template `f.g` cannot deduce function 
from argument types `!()(int)`, candidates are:
/home/per/f.d(11,6):        `g(Arg)(Arg arg)`
   with `Arg = int`
   must satisfy the following constraint:
`       isInputRange!Arg
```

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?

Paul Backus <snarwin gmail.com> writes:

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