http://d.puremagic.com/issues/show_bug.cgi?id=8687

           Summary: Variadic templates do not work properly with default
                    arguments
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-09-18 09:09:36 PDT ---
A simplified version of std.range.lockstep:

auto newLockstep(Args...)(Args args, StoppingPolicy stoppingPolicy =
StoppingPolicy.shortest)
{
     // ...
}

However it doesn't work:

foreach (x, y; newLockstep([1, 2], [1, 2])) { }

test.d(32): Error: template test.newLockstep does not match any
function template declaration
test.d(14): Error: template test.newLockstep(Args...) if
(allSatisfy!(isInputRange,staticMap!(Unqual,Args))) cannot deduce
template function from argument types !()(int[],int[])

Passing an argument in place of the default explicitly does work:
foreach (x, y; newLockstep([1, 2], [1, 2], StoppingPolicy.shortest)) { }

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |k.hara.pg@gmail.com


--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-06-22 00:22:18 PDT ---
A better use-case is:

void foo(Args...)(Args args, string file = __FILE__, size_t line = __LINE__)
{
}

This would allow us to construct error messages with file + line but without creating template bloat because file and line usually have to become template value arguments in order to support variadic arguments. IOW the above currently has to be:

void foo(string file = __FILE__, size_t line = __LINE__, Args...)(Args args)
{
}

But this creates too much template bloat. I've seen other people ask for this feature, I think schveiguy (Steven Schveighoffer) mentioned it as well.

Here's an example use-case, where an enforce-style function can take a format string and avoid the need to call format() explicitly from the call-site:

/** Similar to $(D enforce), except it can take a formatting string as the
second argument. */
T require(T, Args...)
    (T value, lazy Args args, string file = __FILE__, size_t line = __LINE__)
{
    static if (Args.length)
        string msg = Args.length > 1 ? format(args[0], args[1 .. $]) :
text(args);
    else
        enum msg = "requirement failed.";

    if (!value)
        throw new Exception(msg, file, line);

    return value;
}

unittest
{
    string file = "foo.d";
    require(file.exists, "File '%s' does not exist.", file);
}

Currently this won't compile unless you make file and line compile-time arguments.

@Kenji: Do you think it's possible to implement this without a big disruption
(e.g. code breakage)?

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ice


--- Comment #2 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-06-30 10:19:58 PDT ---
Please note that in 2.063 the compiler now ICEs on this unsupported feature:

-----
void foo(T...)(T args, string file = __FILE__) { }

void main()
{
    foo();
}
-----

> Assertion failure: 'index < dim' on line 462 in file 'root\root.h'

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|ice                         |
                 CC|                            |bugzilla@digitalmars.com


--- Comment #3 from Walter Bright <bugzilla@digitalmars.com> 2013-10-06 23:41:37 PDT ---
(In reply to comment #2)
> Please note that in 2.063 the compiler now ICEs on this unsupported feature:
> 
> -----
> void foo(T...)(T args, string file = __FILE__) { }
> 
> void main()
> {
>     foo();
> }
> -----
> 
> > Assertion failure: 'index < dim' on line 462 in file 'root\root.h'

With 2.064 head, produces:

test.d(5): Error: template test.foo does not match any function template
declaration. Candidates are:
test.d(1):        test.foo(T...)(T args, string file = __FILE__)
test.d(5): Error: template test.foo(T...)(T args, string file = __FILE__)
cannot deduce template function from argument types !()()

Removed ice keyword.

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