[Bug tree-optimization/80652] [5 Regression] Union conversion between __m128d and double array does not work under 5.0 through 6.2

[paboyle at ph dot ed.ac.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80652

--- Comment #6 from Peter Boyle <paboyle at ph dot ed.ac.uk> ---
Just a comment -- suggest a warning thrown if union access from
two views is made. AFAIK g++ is the only compiler not implementing the
defacto type pun use.

http://en.cppreference.com/w/cpp/language/union

"Many compilers implement, as a non-standard language extension, the ability to
read inactive members of a union."

i.e. warn on attempted use of the above behaviour.