ref barfs template parameter deduction?

[Paul Backus]

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