What steps will reproduce the problem?
1. A *ast.Object may contain arbitrary data. ast.Fprint assumes it can call Interface()
on arbitrary reflect.Value's The exp/types package injects such data.

2. Call the following file main.go and go run it (you need exp/types)

package main

import (
      "exp/types"
      "go/ast"
      "go/parser"
      "go/token"
      "os"
)

func main() {
      fs := token.NewFileSet()
      file, err := parser.ParseFile(fs, "main.go", nil, 0)
      if err != nil {
            panic(err)
      }

      astpkg, err := ast.NewPackage(fs, map[string]*ast.File{"main.go": file}, types.GcImport, types.Universe)
      if err != nil {
            panic(err)
      }

      _, err = types.Check(fs, astpkg)
      if err != nil {
            panic(err)
      }

      ast.Fprint(os.Stdout, fs, astpkg, nil)
}

What is the expected output? What do you see instead?

Expected: bunches of AST/struct dumps.

Got: an error on a *big.Int:

   501  .  .  .  .  .  .  .  .  .  .  .  .  .  Data: *ast.Scope {
   502  .  .  .  .  .  .  .  .  .  .  .  .  .  .  Outer: nil
   503  .  .  .  .  .  .  .  .  .  .  .  .  .  .  Objects: map[string]*ast.Object (len = 109) {
   504  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  "LEQ": *ast.Object {
   505  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  Kind: const
   506  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  Name: "LEQ"
   507  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  Decl: nil
   508  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  Data: types.Const {
   509  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  val: *panic: reflect.Value.Interface: cannot return value obtained from unexported field or method [recovered]
    panic: interface conversion: interface is string, not ast.localError

goroutine 1 [running]:
go/ast.func·003(0x7f43ab6fff38, 0x7f43ab5e6100)
    /opt/remy/go/src/pkg/go/ast/print.go:52 +0x67
----- stack segment boundary -----
reflect.valueInterface(0x58a4e8, 0xf840118380, 0x161, 0x1, 0x161, ...)
    /opt/remy/go/src/pkg/reflect/value.go:831 +0xe8
reflect.Value.Interface(0x58a4e8, 0xf840118380, 0x161, 0x0, 0x0, ...)
    /opt/remy/go/src/pkg/reflect/value.go:816 +0x42
go/ast.(*printer).print(0xf84011a600, 0x58a4e8, 0xf840118380, 0x161)
    /opt/remy/go/src/pkg/go/ast/print.go:164 +0x7e8
go/ast.(*printer).print(0xf84011a600, 0x4f05e8, 0xf8400d5f50, 0x143)
    /opt/remy/go/src/pkg/go/ast/print.go:145 +0xeb


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