Question

常量表达式只包含常量运算元并且在编译程序时就已经计算完成。

无类型布尔值，数值和字符串常量都可以当作运算元。除了位置操作符，如果二元运算符石不同类型的常量，操作元，和非布尔值，和即将在接下来出现的：整型，rune，浮点数和复数类型。例如：一个无类型整型常量减去无类型复数常量，结果为复数常量。

一个常量的比较运算会生成无类型的布尔常量。如果左移运算是一个无类型常量，结果会是一个整型常量。它会和原来常量为相同类型。其他与无类型常量的运算都会生成相同类型的结果（布尔值，整型，浮点数，复数，字符串常量）。

const a = 2 + 3.0          // a == 5.0   (untyped floating-point constant)
const b = 15 / 4           // b == 3     (untyped integer constant)
const c = 15 / 4.0         // c == 3.75  (untyped floating-point constant)
const Θ float64 = 3/2      // Θ == 1.0   (type float64, 3/2 is integer division)
const Π float64 = 3/2.     // Π == 1.5   (type float64, 3/2. is float division)
const d = 1 << 3.0         // d == 8     (untyped integer constant)
const e = 1.0 << 3         // e == 8     (untyped integer constant)
const f = int32(1) << 33   // illegal    (constant 8589934592 overflows int32)
const g = float64(2) >> 1  // illegal    (float64(2) is a typed floating-point constant)
const h = "foo" > "bar"    // h == true  (untyped boolean constant)
const j = true             // j == true  (untyped boolean constant)
const k = 'w' + 1          // k == 'x'   (untyped rune constant)
const l = "hi"             // l == "hi"  (untyped string constant)
const m = string(k)        // m == "x"   (type string)
const Σ = 1 - 0.707i       //            (untyped complex constant)
const Δ = Σ + 2.0e-4       //            (untyped complex constant)
const Φ = iota*1i - 1/1i   //            (untyped complex constant)

对一个无类型整数，rune，或浮点数应用内置的 complex 函数会生成无类型的复数常量。

const ic = complex(0, c)   // ic == 3.75i  (untyped complex constant)
const iΘ = complex(0, Θ)   // iΘ == 1i     (type complex128)

常量表达式总是一个明确的值；中间值和常量自己可以比语言所支持的精度更高，下面的声明是合法的：

const Huge = 1 << 100         // Huge == 1267650600228229401496703205376  (untyped integer constant)
const Four int8 = Huge >> 98  // Four == 4                                (type int8)

常量的除法的除数不能为 0:

3.14 / 0.0   // illegal: division by zero

定义了类型的常量的精度必须根据常量类型定义。所以下面的常量表达式是非法的：

uint(-1)     // -1 cannot be represented as a uint
int(3.14)    // 3.14 cannot be represented as an int
int64(Huge)  // 1267650600228229401496703205376 cannot be represented as an int64
Four * 300   // operand 300 cannot be represented as an int8 (type of Four)
Four * 100   // product 400 cannot be represented as an int8 (type of Four)

补码使用的一元操作符 ^ 对于非常量的匹配模式：补码对于无符号常量为 1，对于有符号和无类型常量为 -1。

^1         // untyped integer constant, equal to -2
uint8(^1)  // illegal: same as uint8(-2), -2 cannot be represented as a uint8
^uint8(1)  // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
int8(^1)   // same as int8(-2)
^int8(1)   // same as -1 ^ int8(1) = -2

实现限制：编译器在处理无类型浮点数和复数时会取近似值；具体请看常量章节。这个取近似值的操作在浮点数在整数上下文时会产生无效值，即使在计算过后是一个整型。