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x/tools/go/ast/astutil: Apply: extraneous comma after replaced parameter #23771

jba opened this issue Feb 10, 2018 · 0 comments


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commented Feb 10, 2018

Not sure if this is a bug in astutil.Apply or format.Node.
This function:

func applyBug() {
    src := `                                                                                                                                                                                                                                  
package p                                                                                                                                                                                                                                     
import foo "bar"                                                                                                                                                                                                                              
func f(x foo.t) {}                                                                                                                                                                                                                            
    fset := token.NewFileSet()
    file, err := parser.ParseFile(fset, "", src, 0)
    if err != nil {
    pre := func(c *astutil.Cursor) bool {
        if sel, ok := c.Node().(*ast.SelectorExpr); ok {
            if id, ok := sel.X.(*ast.Ident); ok && id.Obj == nil && id.Name == "foo" {
                c.Replace(&ast.Ident{Name: "foo_t"})
        return true
    astutil.Apply(file, pre, nil)
    var buf bytes.Buffer
    if err := format.Node(&buf, fset, file); err != nil {

yields this output:

package p

import foo "bar"

func f(x foo_t,) {}

Note the comma at the end of the function param list.

Reparsing the result with a new FileSet and then re-formatting fixes the problem.

@gopherbot gopherbot added this to the Unreleased milestone Feb 10, 2018

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