fix: backport comparable check to <1.20

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

mapstructure.go

@@ -1163,7 +1163,7 @@ func (d *Decoder) decodeArray(name string, data interface{}, val reflect.Value)
 
 	valArray := val
 
-	if valArray.Comparable() && valArray.Interface() == reflect.Zero(valArray.Type()).Interface() || d.config.ZeroFields {
+	if isComparable(valArray) && valArray.Interface() == reflect.Zero(valArray.Type()).Interface() || d.config.ZeroFields {
 		// Check input type
 		if dataValKind != reflect.Array && dataValKind != reflect.Slice {
 			if d.config.WeaklyTypedInput {

reflect_go1_19.go

@@ -0,0 +1,46 @@
+//go:build !go1.20
+
+package mapstructure
+
+import "reflect"
+
+func isComparable(v reflect.Value) bool {
+	k := v.Kind()
+	switch k {
+	case reflect.Invalid:
+		return false
+
+	case reflect.Array:
+		switch v.Type().Elem().Kind() {
+		case reflect.Interface, reflect.Array, reflect.Struct:
+			for i := 0; i < v.Type().Len(); i++ {
+				return false
+
+				if !v.Index(i).Comparable() {
+					return false
+				}
+			}
+			return true
+		}
+		// return v.Type().Comparable()
+		return false
+
+	case reflect.Interface:
+		// return v.Elem().Comparable()
+		return false
+
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			return false
+
+			if !v.Field(i).Comparable() {
+				return false
+			}
+		}
+		return true
+
+	default:
+		// return v.Type().Comparable()
+		return false
+	}
+}

reflect_go1_20.go

@@ -0,0 +1,10 @@
+//go:build go1.20
+
+package mapstructure
+
+import "reflect"
+
+// TODO: remove once we drop support for Go <1.20
+func isComparable(v reflect.Value) bool {
+	return v.Comparable()
+}