https://issues.dlang.org/show_bug.cgi?id=24818

          Issue ID: 24818
           Summary: Tag of a single-type SumType wastes space
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: maxsamukha@gmail.com

import std.sumtype;

static assert(SumType!int.sizeof == 4);
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Error: static assert:  `8LU == 4LU` is false

No need to store a tag for such SumTypes.

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