https://issues.dlang.org/show_bug.cgi?id=20541

          Issue ID: 20541
           Summary: Digit separator in range format doesn't work
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: simen.kjaras@gmail.com

import std.stdio;
writefln("%,?d", '_', 10000); // A
writefln("%(%,?d%)", [10000]); // B
writefln("%(%,?d%)", '_', [10000]); // C
writefln("%,?d", 10000); // D

On my machine, A prints '10_000', B prints '10,000', C throws 'incompatible format character for integral argument', and D throws 'separator character expected'.

As I have specified in the element format string in B and C, I wish to pass the digit separator as an argument to writefln, but that does not appear possible. Not only that, but D throws, while B doesn't.

Solving this in a way that allows the programmer to pass the digit separator as an argument will require lookahead - the range format specifier will need to know how many arguments the element format specifier expects, and pass it those arguments. If this is not possible, the function should throw on an invalid format specifier as in case D.

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