Syntax question about inlined functions/delegates/lambdas

[Gordon]

Hello,

I have a syntax question regarding the correct usage of 
function/delegates/lambdas, arising after a used it incorrectly 
and it took a long time to debug and see what's going on.
I found out there's a syntax which compiles OK but doesn't work 
(as I naively expected).

The following is a concise example:
===
import std.stdio;
import std.functional;

void call_function(FUNC...)()
{
         alias unaryFun!FUNC _Fun;
         _Fun(42);
}

void main()
{
         call_function!( function(x)    { writeln("funcion,  case 
1. X = ",x); } )();
         call_function!( function(x) => { writeln("funcion,  case 
2. X = ",x); } )();
         call_function!( delegate(x)    { writeln("delegate, case 
3. X = ",x); } )();
         call_function!( delegate(x) => { writeln("delegate, case 
4. X = ",x); } )();
         call_function!( (x)            { writeln("funcion,  case 
5. X = ",x); } )();
         call_function!( (x)         => { writeln("lambda,   case 
6. X = ",x); } )();
         call_function!( (x)         =>   writeln("lambda,   case 
7. X = ",x)    )();
}
===

The output is:
===
$ rdmd ./delegate_question.d
funcion,  case 1. X = 42
delegate, case 3. X = 42
funcion,  case 5. X = 42
lambda,   case 7. X = 42
===

So I've learned that syntaxes in cases 2,4,6 are wrong, but they 
still compile.
May question is - what do they do? what usage do they have (since 
they do not trigger a compilation warning)?

Thanks,
  -gordon