by qslack:

Using go/types' Check on the following program:

package p
import "unicode/utf8"
var foo = utf8.UTFMax

with a types.Context.Ident func results in the func being called for the UTFMax ident.
However, the types.Const object associated with the ident has a nil spec member and
therefore panics when .GetPos() is called on it.

The following test (place in go.exp/go/types/objects_test.go) reproduces this bug:

package types

import (
    "go/ast"
    "go/parser"
    "go/token"
    "testing"
)

func TestConstGetPos(t *testing.T) {
    fset := token.NewFileSet()
    file, _ := parser.ParseFile(fset, "test.go", `package p; import "unicode/utf8"; var foo = utf8.UTFMax`, 0)
    objs := make([]Object, 0)
    c := Context{
        Ident: func(id *ast.Ident, obj Object) {
            objs = append(objs, obj)
        },
    }
    c.Check(fset, []*ast.File{file})
    for _, obj := range objs {
        obj.GetPos() // should not panic
    }
}
 
The panic output is:

$ go test go.exp/go/types
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x8 pc=0x454fc0]

goroutine 7 [running]:
testing.func��004()
    /home/sqs/src/go/src/pkg/testing/testing.go:341 +0xbc
go.exp/go/types.(*Const).GetPos(0xc2003efa50, 0x13)
    /home/sqs/src/go.exp/go/types/objects.go:101 +0x20
go.exp/go/types.TestConstGetPos(0xc200431900)
    /home/sqs/src/go.exp/go/types/objects_test.go:21 +0x247
testing.tRunner(0xc200431900, 0x71fee8)
    /home/sqs/src/go/src/pkg/testing/testing.go:346 +0x8a
created by testing.RunTests
    /home/sqs/src/go/src/pkg/testing/testing.go:426 +0x86b

goroutine 1 [chan receive]:
testing.RunTests(0x644028, 0x71fea0, 0x8, 0x8, 0x5f5001, ...)
    /home/sqs/src/go/src/pkg/testing/testing.go:427 +0x88e
testing.Main(0x644028, 0x71fea0, 0x8, 0x8, 0x7250c0, ...)
    /home/sqs/src/go/src/pkg/testing/testing.go:358 +0x8a
main.main()
    go.exp/go/types/_test/_testmain.go:57 +0x9a
FAIL    go.exp/go/types 0.244s



What is the expected output? Calling obj.GetPos() should return the Pos of the UTFMax
declaration.


What do you see instead? Calling obj.GetPos() causes a panic.


Which compiler are you using (5g, 6g, 8g, gccgo)? 6g


Which operating system are you using? Linux


Which version are you using? go version devel +0ab15b04070e Wed Mar 13 07:45:31 2013
-0700 linux/amd64