On Tuesday, 19 April 2022 at 13:36:26 UTC, Andrey Zherikov wrote:
I want to migrate my library API from standalone function that
takes delegate as argument to a template member function that
takes delegate as a template parameter but compiler errors out.
Here is code example:
```d
import std.stdio;
template T(P)
{
static void f_new(alias func)()
{
func();
}
}
void f(FUNC)(FUNC func)
{
T!int.f_new!(() => func());
}
void main()
{
f(function () { __LINE__.writeln; });
}
```
Compiler error:
```
onlineapp.d(7): Error: `static` function `onlineapp.f!(void
function() @safe).f.f_new!(delegate () @safe
{
(*func)();
return ;
}
).f_new` cannot access delegate `__lambda2` in frame of
function `onlineapp.f!(void function() @safe).f`
onlineapp.d(13): `__lambda2` declared here
onlineapp.d(13): Error: template instance `onlineapp.f!(void
function() @safe).f.f_new!(delegate () @safe
{
(*func)();
return ;
}
)` error instantiating
onlineapp.d(18): instantiated from here: `f!(void
function() @safe)`
```
What confuses me a lot is that if I remove `template T` then
everything works perfectly:
The message is different, but I think this error is probably
related to the "dual context" issue:
https://issues.dlang.org/show_bug.cgi?id=5710
Basically, `f_new!(() => func())` has two scopes that it "wants"
to be nested in: `T!int` and `f!(void function() @safe)`. When
you remove `template T`, there's only one scope, so it's not a
problem.
If you remove `static` from `f_new`, you get an error message
talking about this explicitly:
```
onlineapp.d(5): Deprecation: function `onlineapp.f!(void
function() @safe).f.f_new!(delegate () @safe
{
(*func)();
return ;
}
).f_new` function requires a dual-context, which is deprecated
onlineapp.d(13): instantiated from here: `f_new!(delegate
() @safe
{
(*func)();
return ;
}
)`
onlineapp.d(18): instantiated from here: `f!(void
function() @safe)`
onlineapp.d(13): Error: function `onlineapp.f!(void function()
@safe).f.f_new!(delegate () @safe
{
(*func)();
return ;
}
).f_new` is a nested function and cannot be accessed from
`onlineapp.f!(void function() @safe).f`
```
