Template type inference problem

[Manu]

Here's a clearer presentation of the issue:

void f(T)(const(T)[] x, const(T)* y) {}
void test()
{
    // this works; f(T) is inferred as f!int
    int[] x;
    const int y;
    f(x, &y);

    // fails: expected that f(T) is inferred as f!(int*)
    int*[] a;
    const int* b;
    f(a, &b);
}

Removing `const` from `y` and `b` makes the problem go away, so I
think this is a bug in some interaction between the type deduction
logic and parameter const promotion?


On Thu, 3 Oct 2024 at 09:34, Manu <turkeyman at gmail.com> wrote:

> On Wed, 2 Oct 2024 at 20:06, kdevel via Digitalmars-d <
> digitalmars-d at puremagic.com> wrote:
>
>> On Wednesday, 2 October 2024 at 08:55:15 UTC, Manu wrote:
>> > Does anyone understand why this doesn't work?
>> >
>> > void f(T)(const(T)[] x, const(T)* y) {}
>> > void test()
>> > {
>> >     int*[] x;
>> >     const int* y;
>> >     f(x, &y);
>> > }
>> >
>> > error : template `f` is not callable using argument types
>> > `!()(int*[],
>> > const(int*)*)`
>> >         Candidate is: `f(T)(const(T)[] x, const(T)* y)`
>> >
>> > Should this work? It looks like it should work to me.
>> > ...assuming T is inferred to be `int*`... which it's not clear
>> > why it
>> > wouldn't be?
>> >
>> > The argument `y` is an exact match.
>>
>> The second actual parameter to f!int is &y not y.
>>
>
> Yes, the `y` argument is `const(int*)*`, so `T` must be inferred `int*`
>
> > The argument `x` requires a const promotion,
>>
>> The type of x is int*[] and not const(int) [];
>
>
> Right, `x` is `int*[]`, and matching the argument `const(T)[]` required
> const promotion `int*[]` -> `const(int*)[]`, which is a perfectly normal
> promotion. In which case, `T` is `int*`, matching with `y`, and so `T` can
> be properly inferred.
>
> If you remove the `*` from `x` and `y`, the `T` inference works
> properly... so something about `T` being inferred as `int*` rather than an
> `int` causes the type inference to fail.
> The non-uniformity looks like a bug, unless there's a reasonable
> explanation that I've missed.
>
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