Cannot instantiate ReturnType with lambda

[luafyn]

Why doesn't this work?

```d
import std;

struct Optional(T)
{
     T value;
     bool hasValue;
}

auto map(alias fn, T)(Optional!T m)
{
     alias U = ReturnType!fn;

     if (m.hasValue) {
         return Optional!U(fn(m.value), true);
     } else {
         return Optional!U();
     }
}

void main()
{
     auto x = Optional!string("123", true);
     auto y = x.map!(it => 123);
     writeln(y);
}
```

Error:

```d
onlineapp.d(11): Error: template instance 
`onlineapp.main.ReturnType!(__lambda_L23_C21)` does not match 
template declaration `ReturnType(alias func)`
   with `func = __lambda_L23_C21(__T1)(it)`
   must satisfy the following constraint:
`       isCallable!func`
     alias U = ReturnType!fn;
               ^
onlineapp.d(23): Error: template instance 
`onlineapp.main.map!((it) => 123, string)` error instantiating
     auto y = x.map!(it => 123);
               ^
```

This error message makes noooooo sense to me. At all. It's 
gibberish. How is a lambda not callable?! This is really 
frustrating, as something that works *absolutely trivially* in 
any other language.

It's something that I've ran against with D time and time again: 
I get an inscrutable error or problem that is only solved by 
“just” knowing the right incantation to use, anytime I try to do 
something even slightly unorthodox, and it makes D really 
annoying to use if you’re new to it.

Please ignore the bad encapsulation in the struct, it's just a 
minimal example. Thank you