Stripped down example...

module test;

import std.traits;

auto offsetBy(T,F)(T path, F x, F y)
{
    alias PathFloat = TemplateArgsOf!T[0];

    struct Offseter(Q)
    {
        Q path;
        PathFloat x;
        PathFloat y;
    }

    return Offseter(path, cast(PathFloat) x, cast(PathFloat) y);
}

void Foo()
{
    struct Path(T) { T a; }
    Path!float p;
    offsetBy(p, 1, 1);
}

---->

<source>(18): Error: struct `test.offsetBy!(Path!float, int).offsetBy.Offseter` cannot deduce function from argument types `!()(Path!float, float, float)`, candidates are:

<source>(11):        `test.offsetBy!(Path!float, int).offsetBy.Offseter(Q)`

<source>(25): Error: template instance `test.offsetBy!(Path!float, int)` error instantiating

Compiler returned: 1

(ldc 1.17)

If you look at the template parameters in the error message you'll see "offsetBy!(Path!float, int)"

which is clearly wrong... is it a compiler bug, or just a crappy error message?

On Friday, 15 May 2020 at 21:40:20 UTC, NaN wrote:
> Stripped down example...
>
> module test;
>
> import std.traits;
>
> auto offsetBy(T,F)(T path, F x, F y)
> {
>     alias PathFloat = TemplateArgsOf!T[0];
>
>     struct Offseter(Q)
>     {
>         Q path;
>         PathFloat x;
>         PathFloat y;
>     }
>
>     return Offseter(path, cast(PathFloat) x, cast(PathFloat) y);

This line is the problem. Template argument deduction does not work for constructors; you have to write `Offsetter!T(...)`.

On Friday, 15 May 2020 at 21:55:16 UTC, Paul Backus wrote:
> On Friday, 15 May 2020 at 21:40:20 UTC, NaN wrote:
>> Stripped down example...
>>
>> module test;
>>
>> import std.traits;
>>
>> auto offsetBy(T,F)(T path, F x, F y)
>> {
>>     alias PathFloat = TemplateArgsOf!T[0];
>>
>>     struct Offseter(Q)
>>     {
>>         Q path;
>>         PathFloat x;
>>         PathFloat y;
>>     }
>>
>>     return Offseter(path, cast(PathFloat) x, cast(PathFloat) y);
>
> This line is the problem. Template argument deduction does not work for constructors; you have to write `Offsetter!T(...)`.

Ah OK thanks. Not the most helpful error message from the compiler lol.