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Version: 3.3.7
Platform: 64-bit Windows

It might be a duplicate but I could not find an issue that describes this. Currently, when I have a map of a constant vector type, the return type for the indexing operation of the map becomes a value type instead of a const reference type. This makes the following example not compile.

VectorXd v1(6);
v1 << 1.0, 2.0, 3.0, 4.0, 5.0, 6.0;

const auto vm = Vector3d::Map(&v1[1]);
const auto vmc = Vector3d::Map(&as_const(v1[1]));

cout << &v1(1) << endl;  // okay, pointer to "2.0"
cout << &vm(1) << endl;  // okay, pointer to "3.0"
cout << &vmc(1) << endl; // error, vmc(1) has type "double" instead of "const double&"

Although I can still get the pointer using vmc.data() + 1, the behavior really got me as a surprise. As a comparison with a standard type that does something similar to Map, an std::span of a constant type, when indexed, gives a const reference instead of a value.

My questions are:

1. Is this behavior intended or a bug?
2. If it is intended: is there any reason for this behavior (I thought of striding, alignment, etc, but I cannot convince myself)? If it is a bug: is it possible to correct it?

Thanks!