[Bug tree-optimization/86532] [9 Regression] Wrong code due to a wrong strlen folding starting with r262522

["bernd.edlinger at hotmail dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86532

--- Comment #14 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
(In reply to Martin Sebor from comment #13)
> (In reply to Richard Biener from comment #9)
> > 
> > I bet Martin would argue it's invalid ...
> 
> That's right, the example in comment 8 is undefined because strlen()
> requires a nul-terminated string argument and a[0] in the call strlen(a[0])
> is not such a string.  It's not valid for strlen() to access one subobject
> when passed a pointer to another, even if the two are adjacent in memory. 
> This applies equally to struct members as well as multidimensional arrays.

From a security perspective I cannot agree.

First the sample generates 0 warnings -Wall and -Wextra
Second it generates zero warnings under -fsanitize=address,undefined

This is going into slippery terrain.