Ok given the following code...

auto foo(T)(T x)
{
    struct V1 { T* what; }
    V1 v;
    return v;
}

auto bam(T)(T x)
{
    struct V2 { T* what; }
    V2 v;
    return v;
}

void bar()
{
    bam(foo(1));
}


if you change the declaration of foo or bam to "ref T x", ie..

auto foo(T)(ref T x)
auto bam(T)(ref T x)

then the compiler complains thus...

ldc 1.17.0 (Editor #1, Compiler #1) D#1 with ldc 1.17.0
<source>(23): Error: template `test.foo` cannot deduce function from argument types `!()(int)`, candidates are:

<source>(7):        `test.foo(T)(ref T x)`

Compiler returned: 1

Why would ref make any difference to deciding what function to use, it even says there's only one candidate?

Cheers

On Friday, 8 May 2020 at 22:03:47 UTC, NaN wrote:
> Ok given the following code...
>
> auto foo(T)(T x)
> {
>     struct V1 { T* what; }
>     V1 v;
>     return v;
> }
>
> auto bam(T)(T x)
> {
>     struct V2 { T* what; }
>     V2 v;
>     return v;
> }
>
> void bar()
> {
>     bam(foo(1));
> }

Should have said that compiles fine when x is passed by value.

On Friday, 8 May 2020 at 22:03:47 UTC, NaN wrote:
> void bar()
> {
>     bam(foo(1));
> }
>
>
> if you change the declaration of foo or bam to "ref T x", ie..
>
> auto foo(T)(ref T x)
> auto bam(T)(ref T x)
>
> then the compiler complains thus...
>
> ldc 1.17.0 (Editor #1, Compiler #1) D#1 with ldc 1.17.0
> <source>(23): Error: template `test.foo` cannot deduce function from argument types `!()(int)`, candidates are:
>
> <source>(7):        `test.foo(T)(ref T x)`
>
> Compiler returned: 1
>
> Why would ref make any difference to deciding what function to use, it even says there's only one candidate?
>
> Cheers

The integer literal `1` is an rvalue, and can't be passed by reference.

If you explicitly instantiate the templates foo and bar in the function call, you get a more informative error message:

    bam!int(foo!int(1));

Error: function onlineapp.foo!int.foo(ref int x) is not callable using argument types (int)
       cannot pass rvalue argument 1 of type int to parameter ref int x

On Friday, 8 May 2020 at 22:11:57 UTC, Paul Backus wrote:
> On Friday, 8 May 2020 at 22:03:47 UTC, NaN wrote:
>
> The integer literal `1` is an rvalue, and can't be passed by reference.
>
> If you explicitly instantiate the templates foo and bar in the function call, you get a more informative error message:
>
>     bam!int(foo!int(1));
>
> Error: function onlineapp.foo!int.foo(ref int x) is not callable using argument types (int)
>        cannot pass rvalue argument 1 of type int to parameter ref int x

Ooops facepalm!

Thanks :)