[Bug c++/61004] New: [4.9/4.10 Regression] Spurious warning: dereferencing type-punned pointer

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61004

            Bug ID: 61004
           Summary: [4.9/4.10 Regression] Spurious warning: dereferencing
                    type-punned pointer
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

struct A{};
struct B:A{};
void f(A const&);
int main(){
  B b;
  f(b);
}

g++ -O2 -Wall x.cc -c
x.cc: In function 'int main()':
x.cc:6:6: warning: dereferencing type-punned pointer will break strict-aliasing
rules [-Wstrict-aliasing]
   f(b);
      ^

I think that's perfectly valid C++...
(I don't have a 4.9 handy, I think I've been seeing this since before the
branch, but please check before confirming the bug)