http://d.puremagic.com/issues/show_bug.cgi?id=1951

           Summary: Remove SFINAE
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: jason.james.house@gmail.com


I can imagine the request sounding almost crazy, but it really isn't.
(brief background: SFINAE = substitution failure is not an error.  Commonly
used when selecting which templated definition to apply.  Most uses in C++ are
tricks to determine info available in D is expressions)

I've been polling people on the D mailing list and those I know who are deep into C++, and so far nobody can come up with a real-world use for SFINAE that can't be done with other mechanisms already in D (is expressions, static if, etc...)

What I have been able to find out from people is that SFINAE does cause debugging nightmares with template based code when a specialization fails to compile as expected.

I think trying no SFINAE is a great thing to try in D 2.x while it's still considered experimental/alpha.  I expect a lack of SFINAE to make the compiler simpler and ensure D preserves its fast compile times.

If it does turn out that SFINAE does provide unique functionality (I expect Andrei may know), I think it may be better to enhance existing generic programming techniques than to keep SFINAE.  The further SFINAE gets pushed out of normal use, the more of a corner case it becomes...  biting developers more often than it helps them.


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http://d.puremagic.com/issues/show_bug.cgi?id=1951





------- Comment #1 from wbaxter@gmail.com  2008-03-27 03:26 -------
A good suggestion made on the newsgroups was to first add a switch to D2 for a while that can be used to either disable SFINAE or to print out a trace message every time the compiler uses it.


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http://d.puremagic.com/issues/show_bug.cgi?id=1951



--- Comment #2 from Don <clugdbug@yahoo.com.au> 2012-11-16 00:33:22 PST ---
Here's the first example from http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error translated into D.

---------------
struct Test {
    alias int foo;
}

void f(T)(T.foo x) {} // Definition #1

void f(T)(T x) {}    // Definition #2

void main() {
    f!(Test)(10); // Call #1.
    f!(int)(10);  // C++ No error (even though there is no int::foo) thanks to
SFINAE.
}
------------
bug.d(5): Error: no property 'foo' for type 'int'
bug.d(5): Error: T.foo is used as a type
------------
Same thing with every other SFINAE example I've been able to find. The
behaviour is the same on D1 and D2.
Does D actually have SFINAE???

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--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2012-11-16 00:53:12 PST ---
(In reply to comment #2)
> ---------------
> struct Test {
>     alias int foo;
> }
> 
> void f(T)(T.foo x) {} // Definition #1
> 
> void f(T)(T x) {}    // Definition #2
> 
> void main() {
>     f!(Test)(10); // Call #1.
>     f!(int)(10);  // C++ No error (even though there is no int::foo) thanks to
> SFINAE.
> }
> ------------
> bug.d(5): Error: no property 'foo' for type 'int'
> bug.d(5): Error: T.foo is used as a type
> ------------

I think that is not a lack SFINAE, rather the type deduction mechanism issue.
In current, T.foo is not treated as a *partially parameterized* type name.
I recently filed bug 9022 to support such type deduction.

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Jason House <jason.james.house@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


--- Comment #4 from Jason House <jason.james.house@gmail.com> 2012-12-06 05:51:51 PST ---
I believe Don is right. D does not have SFINAE. Many D users simply assume that was inherited from C++.

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