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digitalmars.D.bugs - [Issue 6662] New: std.functional.memoize with structs


           Summary: std.functional.memoize with structs
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: rejects-valid
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc

--- Comment #0 from bearophile_hugs eml.cc 2011-09-13 04:18:51 PDT ---
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
...\src\phobos\std\functional.d(713):        instantiated from here:
test.d(21):        instantiated from here: memoize!(Foo2)

...\src\phobos\std\traits.d(128): Error: static assert  "argument has no return
...\src\phobos\std\functional.d(713):        instantiated from here:
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;
        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:

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Sep 13 2011