> On Oct 18, 2016, at 12:17 PM, Guoye Zhang via swift-evolution 
> <swift-evolution@swift.org> wrote:
> I propose to ban the top value in Int/UInt which is 0xFFFF... in hex. Int 
> family would lose its smallest value, and UInt family would lose its largest 
> value. Top value is reserved for nil in optionals. An additional benefit is 
> that negating an Int would never crash.

There are two ways to do this (using Int8 for example)
1. 0xFF reserved to mean nil. As this normally means -1, all negative numbers 
now use complements rather than two’s complement form. This breaks a lot of 
binary math.

2. 0x80 reserved to mean nil. This is normally -128. Overflow would have to be 
modified in order to support this (otherwise, 127 + 1 == nil). bit padding no 
longer works (0x80 would expand to 0xFF80 for a Int16 with bit padding, not 

> Interacting with C/Obj-C is a major concern, but since we are already 
> importing some of the unsigned integers as Int which loses half the values, 
> one value is not such big a drawback. Alternatively, we could leave current 
> behavior as CInt/CUInt. Converting them to the new Int?/UInt? doesn't 
> generate any instructions since the invalid value already represents nil.

As the appropriate integer minimum value may already be in use in C or 
Objective C code, I believe you would need to define a new integer types to 
support this sort of constrained type. 

Where I would see something like this be most appropriate would be for 
supporting a “BigNumber” type in the language, preferably as the default 
integer type. Ruby does this for example with Fixnum/Bignum - all values in 
Ruby are actually tagged pointers (where the lower bits are set to cause 
invalid alignment of a pointer in order to indicate it is a special case 
immediate value). So if the lowest bit is set, the value is a FixNum integer 
with a lower max/higher min than a traditional integer. On overflow, the value 
is promoted to be a BigNum, which is a reference to an arbitrary sized integer 
on the heap.

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