Help from the compiler when debugging failing template constraints - a pull request

[Dmitry Olshansky via Digitalmars-d]

On 28-Sep-2015 23:25, Atila Neves wrote:
> I've mentioned this many times before: template constraints are like
> unittest blocks with asserts in D: great that they're built-in easy to
> use. But when they fail, there's no help in figuring out why.
>
[snip]

> I created an input range, verified it compiled, then added a 't' to the
> end of `front`. With `models` I got this for both UDA and static assert
> versions:
>
> /home/atila/coding/d/dlang/phobos/std/range/primitives.d(182): Error:
> template std.range.primitives.front cannot deduce function from argument
> types !()(Foo), candidates are:
> /home/atila/coding/d/dlang/phobos/std/range/primitives.d(2219):
> std.range.primitives.front(T)(T[] a) if (!isNarrowString!(T[]) &&
> !is(T[] == void[]))
> /home/atila/coding/d/dlang/phobos/std/range/primitives.d(2247):
> std.range.primitives.front(T)(T[] a) if (isNarrowString!(T[]))
> /home/atila/coding/d/dlang/phobos/std/traits.d-mixin-6771(6771): Error:
> template instance std.range.primitives.checkInputRange!(Foo) error
> instantiating
> concepts.d(81):        instantiated from here: models!(Foo, isInputRange)
> Failed: ["dmd", "-unittest", "-main", "-v", "-o-", "concepts.d", "-I."]
>
>
> With `static assert(isInputRange!Foo)`, I get this:
>
> concepts.d(87): Error: static assert  (isInputRange!(Foo)) is false
> Failed: ["dmd", "-unittest", "-main", "-v", "-o-", "concepts.d", "-I."]
>
> I prefer the first one. How about you?
>
> Atila

Certainly the first one.


-- 
Dmitry Olshansky