On Fri, Nov 8, 2019 at 11:40 AM André Eriksson <ean...@gmail.com> wrote:
>
> On Friday, November 8, 2019 at 7:16:42 PM UTC+1, Ian Lance Taylor wrote:
>>
>> On Fri, Nov 8, 2019 at 10:06 AM André Eriksson <ean...@gmail.com> wrote:
>> >
>> > Interesting. Do you have a reference to where that happens?
>>
>> The method (*Package).rewriteCall in cmd/cgo/gcc.go. But more useful
>> might be to experiment with some cgo code and build with `go build
>> -work` and look in the $WORK directory to see the generated files.
>>
>>
>> > If i understand you correctly, however, it doesn't appear to solve the
>> > case where the called function fn lives in a different package, and takes
>> > an argument which is a private type. That is:
>> >
>> > -- a/a.go --
>> > package a
>> >
>> > type myString string
>> >
>> > func Fn(str myString) { }
>> >
>> > -- b/b.go --
>> > package b
>> >
>> > import "a"
>> >
>> > func rewrite() {
>> > go a.Fn("some string")
>> > }
>> >
>> > In this example knowing the desired type from the function signature does
>> > not help, I would think?
>>
>> That is correct.
>>
>> Fortunately for a case like that you don't need to use a temporary
>> variable at all, since the argument is a constant.
>>
>> Ian
>
>
> That makes sense. I did some more research and came across another case where
> even this might not work.
> It appears that there is a distinction between an untyped value and an
> untyped constant. I hadn't appreciated this distinction until now.
>
> According to the spec (https://golang.org/ref/spec#Comparison_operators),
> "Comparison operators compare two operands and yield an untyped boolean
> value."
> As a result, it's possible to get untyped, non-constant values from
> non-constant expressions.
>
> So we could have fn(a() == b()) passed to fn(b myBool) with no ability to
> store this argument in a temporary variable without coercing the type to a
> regular typed bool.
Technically true but essentially never happens in practice.
Ian
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