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The literal `-2147483648` is parsed by Metal as negation of positive 2147483648. As 2147483648 is too large for a int, the expression is silently promoted to a long. Sometimes this does not matter as it will often be implicitly converted back to an int after the negation. However, if the expression is used in a bitcast then we hit a compiler error due to mismatched bitwidths. Similarily for `-9223372036854775808`, as 9223372036854775808 is too large for a long, metal emits a `-Wconstant-conversion` warning and changes the value to -9223372036854775808. This would then be negated again, possibly causing undefined behaviour. In both cases we can avoid the issue by expressing the literals as the second most negative value expressible by the type, minus one. eg `-2147483647 - 1` and `-9223372036854775807L - 1L`. We have added a test which uses the most negative i32 literal in an addition. Because we bitcast addition operands to unsigned in metal, this would cause a validation error without this fix. For the i64 case existing tests already make use of the minimum literal value. Passing the flag `-Werror=constant-conversion` to Metal during validation will therefore catch this issue.