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go/types: cannot assign "nil..." to ... parameters #18268

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stjj89 opened this issue Dec 9, 2016 · 2 comments
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go/types: cannot assign "nil..." to ... parameters #18268

stjj89 opened this issue Dec 9, 2016 · 2 comments
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@stjj89
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stjj89 commented Dec 9, 2016

The following code is accepted by both gc and gccgo, but not go/types:

func foo(args ...string) {}

func bar() {
  foo(nil...)
}

The error returned by go/types is

cannot use nil (untyped nil value) as parameter of type []string

It appears that go/types is wrong here. Per the spec
( https://golang.org/ref/spec#Passing_arguments_to_..._parameters, https://golang.org/ref/spec#Assignability ):

If f is variadic with a final parameter p of type ...T, then within f the type of p is equivalent to type []T.
...
If the final argument is assignable to a slice type []T, it may be passed unchanged as the value for a ...T parameter if the argument is followed by .... In this case no new slice is created.

A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases:
...

  • x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type.
@alandonovan
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I don't know why one would ever write that code, but you're right, the spec is pretty clear that it's a legal Go program.

/cc @griesemer

@mdempsky mdempsky added this to the Go1.9 milestone Dec 9, 2016
@gopherbot
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CL https://golang.org/cl/34194 mentions this issue.

@golang golang locked and limited conversation to collaborators Feb 7, 2018
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