[Bug libstdc++/48365] Non-constant references in std::valarray::operator

["paolo.carlini at oracle dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48365

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
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                 CC|denin at mail dot ru        |gdr@integrable-solutions.ne
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--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-03-30 15:29:21 UTC ---
Gaby, can you have a look to this issue? It seems to me that in general, given
the expression templates mechanism we have in place, something like

  k = k[0] * k,        (1)

where k is a valarray, cannot possibly work as intuitively expected, because
the multiplication is expanded "in place": operator= triggers the computation
of the new k[0] and then k[1] which definitely uses the new k[0], contrary to
intuition. Is this actually undefined behavior, like morally in

  operator*(const T& t, const valarray<T>& v)

t cannot be an element of v? Seems something falling under the special features
of valarray wrt aliasing, but I don't see it mentioned anywhere in C++03.

Interestingly, if I change (1) to

  k *= k[0]

which should be in principle equivalent, the behavior is the same on GCC,
whereas another implementation of valarray agrees with GCC in this case.