https://issues.dlang.org/show_bug.cgi?id=23637

          Issue ID: 23637
           Summary: [betterC] DMD issues GC usage error during CTFE
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: jack@jackstouffer.com

The following code does not compile

    char[] unsignedToTempString(uint radix = 10)(ulong value, return scope
char[] buf) @safe
        if (radix >= 2 && radix <= 16)
    {
        size_t i = buf.length;
        do
        {
            uint x = void;
            if (value < radix)
            {
                x = cast(uint)value;
                value = 0;
            }
            else
            {
                x = cast(uint)(value % radix);
                value /= radix;
            }
            buf[--i] = cast(char)((radix <= 10 || x < 10) ? x + '0' : x - 10 +
'a');
        } while (value);
        return buf[i .. $];
    }

    char[] myToString(ulong n)
    {
        char[20] buf;
        auto s = unsignedToTempString(n, buf);
        return s ~ (n > uint.max ? "UL" : "U");
    }

    enum a = myToString(5);

Fails with

Error: array concatenation of expression `cast(const(char)[])s ~ (n >
4294967295LU ? "UL" : "U")` requires the GC which is not available with
-betterC

--