Overflow error when converting negative int64 to uint64 in one liner

[Ugorji Nwoke]

http://play.golang.org/p/WSqe7NtkiW

Why can I do:

var x int64 = -1

var y uint64 = uint64(x)

But cannot do:

var y2 uint64 = uint64(int64(-1))

For the second one, I get:

constant -1 overflows uint64

Is this a bug? I spent a while trying to figure out why I was getting the error. From my limited understanding, it seems that the conversion first to an int64 is stripped out when done on one line.

[David Symonds]

Sounds like a bug to me. File something on the issue tracker.

[Ugorji Nwoke]

Thanks. 

Filed: https://code.google.com/p/go/issues/detail?id=5299

[Daniel Theophanes]

The first case happens at run-time. The compiler doesn't try to prove every path.

The second case is evaluated at compile time. When the compiler evaluates constants, it does checks like these even if their run-time behavior is different.

[Ugorji Nwoke]

David just responded to the issue, and it seems this has to do with typed constants. 

var v int64 = -1 // v is now a variable

var v2 uint64 = uint64(v) // converting a variable of type int64 to uint64 is allowed, 

var v2 uint64 = uint64(int64(-1)) // the int64(-1) is a typed constant, and cannot legally be converted to a uint64 

This and other examples are in the spec at: http://tip.golang.org/ref/spec#Constant_expressions

So this doesn't seem like a bug. 

Thanks David.