Go fails to infer type in assignment: "non-name on left side of :="

This snippet works as expected play.golang.org/p/VuCl-OKMav

i := 10
next := 11
prev, i := i, next

However this nearly identical snippet gives non-name f.Bar on left side of := play.golang.org/p/J8NNWPugQG

type Foo struct {
    Bar int
}

f := Foo{10}
next := 11
prev, f.Bar := f.Bar, next

What's special about the struct that stops type inference? Is this a bug?

Solution 1:

It's an open issue.

Issue 6842: spec: Assigning to fields with short declaration notation

Solution 2:

It's not really a type inference issue, it's just that the left-hand-side of := must be a list of identifiers, and f.Bar is not an identifier, so it can't be declared — not even with :='s slightly-more-permissive rules for what it can declare. See "Short variable declarations" in The Go Programming Language Specification.

Solution 3:

From the spec's Short variable declarations section:

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block...with the same type, and at least one of the non-blank variables is new.

So if you declare the variable inside another type (struct Foo in the example), it is disqualified by "provided they were originally declared earlier in the same block".

So the answer is to just set the pre-declared variable equal not using the := syntax to the value:

...
var prev int
prev, f.Bar = f.Bar, next
...