typeof function literals which define the types of its parameters but do not give their parameters names

[Paul Backus]

On Thursday, 27 December 2018 at 03:52:52 UTC, Johannes Loher 
wrote:
> Hey all,
>
> I am a bit confused about the inferred types of function 
> literals which do not name their parameters (something like 
> `(int) {}`). The confusion arises from the fact that the 
> inferred type sometimes is `void` (might be the case, because 
> the function literal is inferred to be a template) and 
> sometimes it is something like `void function(bool _param_0) 
> pure nothrow @nogc @safe`. What is really weir is that seems to 
> depend on the type of the parameter. Here is small example 
> showing what happens for different parameter types: 
> https://run.dlang.io/is/xSMZZu
>
> Also note that this only happens for function literals. For 
> regular functions and member functions, `void` is never 
> inferred, but only types like `pure nothrow @nogc @safe 
> void(MyClass _param_0)`.
>
> Has anybody any idea what is going on here? Is this a bug?

As you suspect, some of the function literals are being 
interpreted as templates--specifically, the ones whose parameters 
are parsed as "identifiers" rather than built-in types. Keep in 
mind that from the parser's perspective, `string`, `size_t`, and 
`Object` aren't any different from other user-defined type names 
or aliases. That they happen to be defined in the D runtime is 
irrelevant.

You can see that this is true by running the following example:

     alias f = (string) {  };
     writeln(typeof(f!int).stringof);

Unfortunately, this behavior is mentioned exactly nowhere in the 
language spec. Both the section on function literals [1] and the 
grammar for function parameters to which it refers [2] fail to 
mention any circumstance in which a parameter declaration may 
consist only of a name, without a type. As with many things in D, 
one can only figure out what the *actual* rules are through trial 
and error.

[1] https://dlang.org/spec/expression.html#function_literals
[2] https://dlang.org/spec/function.html#grammar