In the following D2 program only the Mfoo1 compiles:


import std.functional: memoize;
struct Foo1 {
    int x;
    static Foo1 opCall(int x_) {
        Foo1 f;
        f.x = x_;
        return f;
    }
}
struct Foo2 {
    int x;
}
struct Foo3 {
    int x;
    this(int x_) {
        this.x = x_;
    }
}
void main() {
    alias memoize!Foo1 Mfoo1;
    alias memoize!Foo2 Mfoo2;
    alias memoize!Foo3 Mfoo3;
}


DMD 2.055 gives errors like:

...\src\phobos\std\traits.d(128): Error: static assert  "argument has no return type"
...\src\phobos\std\functional.d(713):        instantiated from here: ReturnType!(Foo2)
test.d(21):        instantiated from here: memoize!(Foo2)

...\src\phobos\std\traits.d(128): Error: static assert  "argument has no return type"
...\src\phobos\std\functional.d(713):        instantiated from here: ReturnType!(Foo3)
test.d(22):        instantiated from here: memoize!(Foo3)


The static assert is in std.traits:

template ReturnType(/+@@@BUG4217@@@+/func...)
    if (/+@@@BUG4333@@@+/staticLength!(func) == 1)
{
    static if (is(FunctionTypeOf!(func) R == return))
        alias R ReturnType;
    else
        static assert(0, "argument has no return type");
}


I think Mfoo2 and Mfoo3 too should compile. Writing Foo2(5) is like calling the function Foo2 with argument x = 5, and returning a struct Foo2 result. I think this is a place where telling apart structs and functions is an artificial distinction that is the opposite of useful.


See one use case:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=144333