Fix panic in DefaultResolveFn if uses the string type alias in source…

… key

This closes graphql-go#700.

executor.go

@@ -995,7 +995,20 @@ func DefaultResolveFn(p ResolveParams) (interface{}, error) {
 
 	// Try accessing as map via reflection
 	if r := reflect.ValueOf(p.Source); r.Kind() == reflect.Map && r.Type().Key().Kind() == reflect.String {
-		val := r.MapIndex(reflect.ValueOf(p.Info.FieldName))
+		fieldNameValue := reflect.ValueOf(p.Info.FieldName)
+		mapKeyType := r.Type().Key()
+		// The map key type might be a string type alias and its underlying type is string,
+		// but it will be panic if we try to use it as a string value in `MapIndex`.
+		// So we need to convert the value of the field name to the map key type before
+		// using it as a map key.
+		//
+		// Related issue: https://github.com/graphql-go/graphql/issues/700
+		if !fieldNameValue.CanConvert(mapKeyType) {
+			return nil, nil
+		}
+		fieldNameValue = fieldNameValue.Convert(mapKeyType)
+
+		val := r.MapIndex(fieldNameValue)
 		if val.IsValid() {
 			property := val.Interface()
 			if val.Type().Kind() == reflect.Func {

executor_test.go

@@ -15,6 +15,17 @@ import (
 	"github.com/graphql-go/graphql/testutil"
 )
 
+func TestDefaultResolveFn(t *testing.T) {
+	type Key string
+	type Source map[Key]interface{}
+	source := Source{
+		"foo": "bar",
+	}
+	graphql.DefaultResolveFn(graphql.ResolveParams{
+		Source: source,
+	})
+}
+
 func TestExecutesArbitraryCode(t *testing.T) {
 
 	deepData := map[string]interface{}{}