当变量可以时，不能将int32 const强制转换为uint32的原因是什么？ [重复]

Question

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• Does Go compiler's evaluation differ for constant expression and other expression 1 answer

I've found that you can cast a variable int32 to uint32 but you cannot cast a const int32 to uint32.

See here: https://play.golang.org/p/tDm6B6g5P6u

If line 14 is commented out it works.

Does anyone have an explanation for this?

Thanks!

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Answer 1

The expression uint32(ci) is a constant expression. The spec says this about constant expressions:

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

A uint32 cannot accurately represent the negative value ci, therefore this expression results in a compilation error.

Positive values in range are supported. For example, uint32(-ci) compiles with no error.

The expressions uint32(vi) and uint32(vc) are conversions. Conversions betweeen numeric types are allowed, even when there's loss of accuracy.