Any integer type can convert to match this signature, but the diagnostic
will only report that this deleted function was matched. Switch to a
template function so that the diagnostic can report the unconverted
integer type.
`std::size_t` is `unsigned int` on i686-pc-linux-gnu, but is `unsigned
long` on x86_64-pc-linux-gnu. This mismatch allows d_array<E =
std::size_t> to be well-formed on x86_64, but trigger a duplicate
definition of operator[](E) on i686. Add a requires() check that
forbids both types for E, so that code which would break the i686 build
is also diagnosed in the x86_64 build.
These are not necessary for proper compilation, but are included so that
a mismatch between the forward declaration and the definition will be
diagnosed immediately.
This ensures that an array is indexed by the appropriate type. Indexing
a mouse array with a joystick index is undefined behavior, and with this
change, such undefined behavior is now a compilation error.
