The number 0 happily and unambiguously plays a role in many numeric types and algebraic structures, and it’s kind of nice if the compiler can just figure out the type that it should be from context. Comparing a literal 0 to a double should cast to a double, comparing a literal 0 to an int should cast to an int, etc. I would be happy for “(int) 0 == (double) 0” to raise a type error though.

I guess it’s a matter of interpretation: does “x == 0” mean that we are comparing x with the integer zero, or the “zero value” of the same type as x? Numeric code is difficult for many reasons, but something that can make it much more tractable is having the code as close as possible to the mathematics underlying the algorithms, and this kind of “polymorphic constant” behaviour can help a lot, especially for integer literals which unambiguously embed into essentially any numeric type you could think of.