CTFE bug causes null check to pass on null pointers (Issue 7602)

[H. S. Teoh]

While testing my AA implementation on existing AA-related bug, I came
across this issue:

	http://d.puremagic.com/issues/show_bug.cgi?id=7602
 
Upon playing around a bit with the sample code given in the bug, I
managed to find a code snippet that would cause this code:

	if (impl !is null) { ... }

to actually execute the code inside the if-statement body ***even when
impl is null***, while running in CTFE.

I've bumped the severity to critical because something is very very
wrong with this.


T

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