[Issue 22091] New: Unexpected behaviour with variadic template param followed by default parameter

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22091

          Issue ID: 22091
           Summary: Unexpected behaviour with variadic template param
                    followed by default parameter
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: sharonh at weka.io

When using a variadic template parameter in a function, it is possible to
define another parameter afterwards. This behaves in the expected way in most
cases, however when the parameter is a default parameter, it is impossible to
pass anything into it, since the variadic parameter seems to be greedy and
always takes in whatever is passed to it.

Here's an example that shows this behaviour:

import std;
void variadic(Args...)(Args args) {
    writefln("variadic - Args len is %s", args.length);
    variadic2(args, 5);
}
void variadic2(Args...)(Args args, int anotherArg = 4) {
    writefln("variadic2 - Args len is %s, args is %s, anotherArg is %s",
args.length, args, anotherArg);
}
void main()
{
    variadic();
}

This example prints (seems to be consistent across compilers and operating
systems):

variadic - Args len is 0
variadic2 - Args len is 1, args is 5, anotherArg is 4

This is counterintuitive behaviour. I think the correct behaviour in this case
would be for it to not compile.

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