isCallable fails

[Paul Backus]

On Wednesday, 20 January 2021 at 04:43:12 UTC, frame wrote:
> struct foo(T) {
>   T get() {
>         static if (is(T : bar)) {
>             if (value is null) {
>                 value = fun!T;

Error: template instance `fun!T` template `fun` is not defined

Please post an example with enough code to actually produce the 
error you're seeing.