https://issues.dlang.org/show_bug.cgi?id=23643

          Issue ID: 23643
           Summary: [betterC] Better Error Message For CTFE GC Usage
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: jack@jackstouffer.com

Consider the following real code example,


    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);

This gives the following error

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

The problem here is that DMD is trying to include the function myToString into the object file even though it's only being used during CTFE. The fix for this code is to make myToString a template. But the chance that a user is going to know that is very low.

The compiler knows that it encountered this function during the CTFE run. So it should give a helpful error message, something like.

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

    This GC allocation was encountered during CTFE. If this function is
supposed to be a CTFE only function then this error can be fixed by making the
function a template.

Simple, easy to understand, and very helpful.

--