fix for issue #1634 -- for passing a closure to exported Go function

This fixes issue #1634 

includes special case for defer function.

I could remove or significantly reduce the comment description, and just have that here for future reference:

// per #1634, if v is already a func, then don't re-wrap!  critically, the original wrapping
// clones the frame, whereas the one here (below) does _not_ clone the frame, so it doesn't
// generate the proper closure capture effects!
// this path is the same as genValueAsFunctionWrapper which is the path taken above if
// the value has an associated node, which happens when you do f := func() ..

interp/interp_issue_1634_test.go

@@ -0,0 +1,59 @@
+package interp
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"reflect"
+	"testing"
+)
+
+func TestExportClosureArg(t *testing.T) {
+	outExp := []byte("0\n1\n2\n")
+	// catch stdout
+	backupStdout := os.Stdout
+	defer func() {
+		os.Stdout = backupStdout
+	}()
+	r, w, _ := os.Pipe()
+	os.Stdout = w
+
+	i := New(Options{})
+	err := i.Use(Exports{
+		"tmp/tmp": map[string]reflect.Value{
+			"Func": reflect.ValueOf(func(s *[]func(), f func()) { *s = append(*s, f) }),
+		},
+	})
+	if err != nil {
+		t.Error(err)
+	}
+	i.ImportUsed()
+
+	_, err = i.Eval(`
+func main() {
+	fs := []func(){}
+	
+	for i := 0; i < 3; i++ {
+		i := i
+		tmp.Func(&fs, func() { println(i) })
+	}
+	for _, f := range fs {
+		f()
+	}
+}
+`)
+	if err != nil {
+		t.Error(err)
+	}
+	// read stdout
+	if err = w.Close(); err != nil {
+		t.Fatal(err)
+	}
+	outInterp, err := io.ReadAll(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(outInterp, outExp) {
+		t.Errorf("\nGot: %q,\n want: %q", string(outInterp), string(outExp))
+	}
+}

interp/run.go

@@ -987,7 +987,20 @@ func genFunctionWrapper(n *node) func(*frame) reflect.Value {
 	}
 	funcType := n.typ.TypeOf()
 
+	value := genValue(n)
+	isDefer := false
+	if n.anc != nil && n.anc.anc != nil && n.anc.anc.kind == deferStmt {
+		isDefer = true
+	}
+
 	return func(f *frame) reflect.Value {
+		v := value(f)
+		if !isDefer && v.Kind() == reflect.Func {
+			// fixes #1634, if v is already a func, then don't re-wrap
+			// because original wrapping cloned the frame but this doesn't
+			return v
+		}
+
 		return reflect.MakeFunc(funcType, func(in []reflect.Value) []reflect.Value {
 			// Allocate and init local frame. All values to be settable and addressable.
 			fr := newFrame(f, len(def.types), f.runid())