confusion about convert `uint8` to `int8`

Question

I want to convert uint8 to int, so I write a const 0xfc, and try to use int8(0xfc) to convert it. However the code raises

Answer 1

Is there any difference between these two codes?

The first example uses a constant expression. The second uses plain expressions. Constant expressions are evaluated at compile time with different rules from plain expressions.

Why does the first one raise a compile error?

The int8(0xfc) is a typed constant expression. Values of typed constants must always be accurately representable by values of the constant type. The compiler reports an error because the value 252 cannot be represented by the values of int8.

Based on comments on other answers, I see that the goal is to get an int32 from a byte with sign extension. Given a byte variable b, use the expression int32(int8(b)) to get the int32 value with sign extension.