https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63645
--- Comment #19 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- Given GnmExprBinary res; GnmExpr const *expr = (GnmExpr *)&res; the C standard does not define where the result of the conversion points; all that's defined is that if converted directly back to GnmExprBinary * (not via some sequence of intermediate types) it compares equal to the original pointer, if the original pointer was sufficiently aligned for GnmExpr * (and it is likely the latter type has stricter alignment requirements than GnmExprBinary *). The effect is that the pointer resulting from the conversion cannot be dereferenced, only compared and converted back. It is these limits on what pointer conversions are defined, together with the direct rules on what types of lvalues may access an object, that restrict aliasing in C.
