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spec: legal conversions from string constant to byte slice are not covered by convertability rules #25991

octomarat opened this issue Jun 21, 2018 · 2 comments


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@octomarat octomarat commented Jun 21, 2018

According to Conversions (as of 21.06.2018), the following programs should be illegal, but it compiles without errors:

package main

func main() {
	_ = []byte("s") // no compile time error

Here we have constant value "s" converted to []byte. According to spec, "s" must be Representable by []byte. The only way to explain the absence of error is to suppose that "s" is in the set of values determined by []byte.

The assumption above contradicts with Assignability rules. Consider the following program that fails to compile:

package main

func main() {
	var b []byte
	b = "s" // compile time error

So, "s" is not assignable to []byte. As a consequence, "s" is not representable by []byte and "s" is not in the set of values determined by []byte.

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@bradfitz bradfitz commented Jun 21, 2018

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@octomarat octomarat commented Jun 21, 2018

Yes, this is the part that covers it, thanks.

However, this is what Conversions section says for constant values:

A constant value x can be converted to type T if x is representable by a value of T. As a special case, an integer constant x can be converted to a string type using the same rule as for non-constant x.

As you can see, conversion from integer constant to string is noted as a special case and is governed by the rules for non-constant x. These rules are described in the section that I missed (Conversions to and from a string type).

But that makes me think that the Conversions to and from a string type is for non-constant values. The section itself does not state explicitly what values the rules are applied to. As a result, it is not obvious that I should consult that section when working with constant values.

Moreover, Conversions section contains the following note:

Specific rules apply to (non-constant) conversions between numeric types or to and from a string type. These conversions may change the representation of x and incur a run-time cost. All other conversions only change the type but not the representation of x.

Again, as far as I can see it is about Conversions to and from a string type.

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