Why does converting a uint8 constant to int8 directly cause a compile error, but it works if the conversion is done after assignment?

Confusion about Converting uint8 to int8

Question:

The following code attempts to convert a uint8 constant to an int8, but encounters a compile error:

<code class="go">package main

func main() {
    a := int8(0xfc)  // compile error: constant 252 overflows int8
    _ = a
}</code>

On the other hand, the following code does not raise the error if you defer the type conversion after the assignment:

<code class="go">package main

func main() {
    a := 0xfc
    b := int8(a)  // ok
    _ = b
}</code>

What is the difference between these two codes? Also, why does the first code give a compile error?

Answer:

In the first code, the constant 0xfc is evaluated before converting to type int8. 0xfc is a value that fits in uint8, but not int8. Therefore, the compiler generates the error "constant 252 overflows int8".

In the second code, the type conversion occurs after the assignment is made. This evaluates a as a uint8 type and then converts it to an int8 type. Therefore, no error occurs.

Conclusion:

If you want to convert uint8 to int8, you need to make sure that the value of the constant is within the range of int8 before converting. Otherwise, a compilation error will occur.

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