Why int64(1.1*float64(time.Minute)) is an error while int64(0.5*float64(time.Minute)) is not in golang 1.9.2?

Question

I meet a very strange bug in golang 1.9.2: When I try to write int64(1.1*float64(time.Minute)) shows an error. The compiler said that the constant truncated to integer.

But when I change 1.1 to other float like 1.2 0.5 1.7, it compiles!

And it can also compile when I write it like:

value:=1.1*float64(time.Minute)
fmt.Println(int64(value))

Is it some bug of go itself? I run go on ubuntu14.04 x64

Answer 1

The constant 1.1*float64(time.Minute) has a fractional part (the value is approximately 6.600000000000001e+10).

The specification says this about constant expressions :

The values of typed constants must always be accurately representable by values of the constant type.

The constant expression int64(1.1*float64(time.Minute)) fails to compile because an int64 cannot represent the number because it has a fractional part.

The values 1.2 * float64(time.Minute) , 0.5 * float64(time.Minute) and 1.7 * float64(time.Minute) can be converted to an int64 in a constant expression because these values do not have a fractional part.

The following snippet compiles because the conversion from the variable value to int64 is not a constant expression.

value:=1.1*float64(time.Minute)
fmt.Println(int64(value))