On 4/19/22 11:46 AM, Paul Backus wrote:
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.
No, there is no context pointer necessary for a template instantiation,
without one being artificially introduced via an alias parameter.
`T!int` is essentially just a namespace, especially since the `P`
parameter isn't even used.
If you remove `static` from `f_new`, you get an error message talking
about this explicitly:
Interesting that `static` does anything there, since it's a no-op.
-Steve
