In Go, why does a[1:] not give an index out of bounds error for a capacity = 1 slice?

Question

Why does the following code not give a "slice bounds out of range" error?

a := []int{0}
a = a[1:]
fmt.Println(a) // []

Answer 1

Because the Go specification for slice expressions states:

For a string, array, pointer to array, or slice a, the primary expression

a[low : high]

constructs a substring or slice.

...

For convenience, any of the indices may be omitted. A missing low index defaults to zero; a missing high index defaults to the length of the sliced operand:

a[2:] // same as a[2 : len(a)]

...

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range.

In your case, len(a) is 1, and a[1:] is the same as a[1:1], which means it is within range.