https://issues.dlang.org/show_bug.cgi?id=21738

          Issue ID: 21738
           Summary: std.format.spec: singleSpec should throw on "%%"
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: bugzilla@bernis-buecher.de

singleSpec checks the format specifier and throws if it is invalid, but forgets to throw on %%.

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