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text/template: type checker bug with missing map keys #3850

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remyoudompheng opened this issue Jul 22, 2012 · 4 comments
Closed

text/template: type checker bug with missing map keys #3850

remyoudompheng opened this issue Jul 22, 2012 · 4 comments
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@remyoudompheng
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@remyoudompheng remyoudompheng commented Jul 22, 2012

What steps will reproduce the problem?
1. Compile and run the following program

package main

import (
      "bytes"
      "fmt"
      "math/big"
      "text/template"
)

func main() {
      n, _ := new(big.Int).SetString("123456789", 10)
      display := func(n *big.Int) string { return n.String() }
      tpl := template.Must(template.
            New("test").
            Funcs(template.FuncMap{"display": display}).
            Parse(`{{ index $.Values $.Key | display }}`))

      type data struct {
            Values map[string]*big.Int
            Key    string
      }

      values := map[string]*big.Int{"number": n, "hello": nil}

      for _, key := range []string{"number", "hello", "world"} {
            buf := new(bytes.Buffer)
            err := tpl.Execute(buf, data{values, key})
            fmt.Printf("%q\n%v\n", buf.String(), err)
      }
}

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

Expected:
"123456789"
<nil>
"<nil>"
<nil>
"<nil>"
<nil>

Got:
"123456789"
<nil>
"<nil>"
<nil>
""
template: test:1: wrong type for value; expected *big.Int; got string

Please use labels and text to provide additional information.
@remyoudompheng
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@remyoudompheng remyoudompheng commented Jul 22, 2012

Comment 1:

I expect the following change but it breaks the tests.
diff -r 5e7fd762f356 src/pkg/text/template/funcs.go
--- a/src/pkg/text/template/funcs.go  Tue Jul 17 07:56:25 2012 +0200
+++ b/src/pkg/text/template/funcs.go  Sun Jul 22 12:07:02 2012 +0200
@@ -128,7 +128,7 @@
       if x := v.MapIndex(index); x.IsValid() {
         v = x
       } else {
-        v = reflect.Zero(v.Type().Key())
+        v = reflect.Zero(v.Type().Elem())
       }
     default:
       return nil, fmt.Errorf("can't index item of type %s", index.Type())
@robpike
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@robpike robpike commented Jul 23, 2012

Comment 2:

Labels changed: added priority-soon, packagebug, removed priority-triage.

Owner changed to @robpike.

Status changed to Accepted.

@robpike
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@robpike robpike commented Jul 23, 2012

Comment 3:

Your fix is correct; there is an incorrect test that should have caught this.

Status changed to Started.

@robpike
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@robpike robpike commented Jul 23, 2012

Comment 4:

This issue was closed by revision ce27433.

Status changed to Fixed.

robpike added a commit that referenced this issue May 11, 2015
««« backport 0748cd92ed76
text/template: fix bug in map indexing
If the key is not present, return value of the type of the element
not the type of the key. Also fix a test that should have caught this case.

Fixes #3850.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6405078

»»»
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