Expression tuples cannot be used for indexing built-in arrays:
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import std.typetuple;
void main()
{
    alias TypeTuple!(1) index;
    auto arr = new int[4];
    auto x = arr[index];    // error
}
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% dmd -o- -c test
test.d(6): Error: cannot implicitly convert expression (tuple(1)) of type (int) to uint
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While overloaded opIndex accepts expression tuple.
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import std.typetuple;
void main()
{
    alias TypeTuple!(1) index;
    S s;
    auto x = s[index];      // okay
}
struct S
{
    size_t opIndex(size_t i) { return i; }
}
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