https://issues.dlang.org/show_bug.cgi?id=13441

          Issue ID: 13441
           Summary: joiner asserts with only(x) separator
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Phobos
          Assignee: nobody@puremagic.com
          Reporter: thecybershadow@gmail.com

/////////// test.d ///////////
import std.algorithm;
import std.range;

void main()
{
    string[][] lines = [null];
    lines
        .joiner(only("b"))
        .array;
}
//////////////////////////////

This trips the "internal error" assertion in joiner.Result.useSeparator.

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