[Issue 20964] poor CTFE support for backward pointer iteration

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20964

--- Comment #3 from Walter Bright <bugzilla at digitalmars.com> ---
To elaborate, decrementing past the beginning could lead to a wraparound, which
is why this restriction is in the memory model.

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