[Issue 17953] inout-like mechanism to infer function attributes from callback attributes

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17953

--- Comment #1 from anonymous4 <dfj1esp02 at sneakemail.com> ---
Apparently even templates don't help here much:
---
import std.typecons:Nullable;
Nullable!V convert(T,V)(Nullable!T v, scope V delegate(T) c)
{
        if(v.isNull)return Nullable!V();
        return Nullable!V(c(v.get()));
}

void f() pure
{
        Nullable!long v;
        Nullable!int v1=v.convert!(long,int)((a){ return cast(int)a; });
}
---
Error: pure function 'f' cannot call impure function 'convert!(long,
int).convert'

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