clang warns for using an uninitialized array during the
member-initialization-list, before the union constructor would have done
nothing. clang permits using the still-uninitialized array in the
constructor body, after the union constructor has done nothing.
The same code is generated both before and after this commit, but the
old code produces a warning and the new code is silent.
```
similar/main/physics.cpp:282:5: error: field 'a' is uninitialized when used here [-Werror,-Wuninitialized]
e(a.begin())
```
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