Untyped `X` is a rune constant (in other words, its default type is `rune`, a.k.a `int32`, a 32-bit integer type).
Untyped `Y` is an int constant (in other words, its default type is `int`, a 64-bit integer type on 64-bit OSes).
At run time, the expression `A << n` overflows, so it is evaluated as 0;
on the other hand, the expression `B << n` doesn't overflow.
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