Using operator= is not quite sound in presence of destructors and operator
overloading.
It's perfectly fine to assume that the left-hand-side of an operator= expression
is valid memory, however we're using uninitialized memory here, that may not be
the case.
Use placement new to properly construct tagged unions. I don't need this with
any urgency, but it's the right thing to do in presence of complex types, and
the current code seems a bomb waiting to explode :)
Opt-in since it uses a C++17 feature, but this allow exactly the same usage in
C++ and Rust, which I think is nice.
This also fixes constants of transparent structs in general, since the initial
version of this patch broke a test (associated constants in enums), so I added a
test for that too.
Emilio Cobos Álvarez
Joshua Groves
Dan Robertson
Vincent Esche