Can't operator == be applied to generic types in C#?

Question

According to the documentation of the == operator in MSDN,

For predefined value types, the equality operator (==) returns true if th

Answer 1

It appears that without the class constraint:

bool Compare (T x, T y) where T: class
{
    return x == y;
}

One should realize that while class constrained Equals in the == operator inherits from Object.Equals, while that of a struct overrides ValueType.Equals.

Note that:

bool Compare (T x, T y) where T: struct
{
    return x == y;
}

also gives out the same compiler error.

As yet I do not understand why having a value type equality operator comparison is rejected by the compiler. I do know for a fact though, that this works:

bool Compare (T x, T y)
{
    return x.Equals(y);
}