fix: apply method receiver offset when generating interface wrapper

Fixes #772.

_test/issue-772.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"log"
+	"os"
+	"text/template"
+)
+
+type Message struct {
+	Data string
+}
+
+func main() {
+	tmpl := template.New("name")
+
+	_, err := tmpl.Parse("{{.Data}}")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	err = tmpl.Execute(os.Stdout, Message{
+		Data: "Hello, World!!",
+	})
+
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Output:
+// Hello, World!!

interp/run.go

@@ -1080,7 +1080,7 @@ func callBin(n *node) {
 	}
 
 	for i, c := range child {
-		defType := funcType.In(pindex(i, variadic))
+		defType := funcType.In(rcvrOffset + pindex(i, variadic))
 		switch {
 		case isBinCall(c):
 			// Handle nested function calls: pass returned values as arguments