Why Does Go's Type Inference Fail for Struct Fields in Short Variable Declarations?

Go Type Inference Ambiguity in Assignment with Structs

Consider the following Go code snippet:

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

In this snippet, the type of i is successfully inferred as int during assignment. However, when a similar snippet involves a struct field, the type inference fails, as illustrated below:

type Foo struct {
    Bar int
}

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

In the latter snippet, the type inference for f.Bar fails with the error message "non-name f.Bar on left side of :=".

To explain this behavior, we turn to the relevant issue in the Go tracker:

Github Issue 6842

Issue 6842 ("spec: Assigning to fields with short declaration notation") documents this behavior as an outstanding issue. The problem stems from the ambiguity in the syntax of the assignment. Specifically, the compiler cannot determine whether f.Bar is intended to be a variable name or a field name within the expression f.Bar, next.

While the issue report marks it as open, later comments suggest that it may have been resolved or superseded by other changes. However, the latest official word remains that this is a known issue.

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