feat: partially revert #1387

1387 introduced a rather serious breaking change that's causing pain
throughout the community. Even though it fixed a buggy behavior
the previous one was still useful to a lot of people.

This change introduces a way to revert back to that old behavior.

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

internal/features/1387.go

@@ -0,0 +1,7 @@
+//go:build !viper_1387
+// +build !viper_1387
+
+package features
+
+// Revert1387 reverts the behavior introduced in #1387.
+var Revert1387 = false

internal/features/1387_.go

@@ -0,0 +1,7 @@
+//go:build viper_1387
+// +build viper_1387
+
+package features
+
+// Revert1387 reverts the behavior introduced in #1387.
+var Revert1387 = true

internal/features/doc.go

@@ -0,0 +1,2 @@
+// Package features allows toggling features in Viper based on build tags.
+package features

util.go

@@ -19,6 +19,8 @@ import (
 	"unicode"
 
 	"github.com/spf13/cast"
+
+	"github.com/spf13/viper/internal/features"
 )
 
 // ConfigParseError denotes failing to parse configuration file.
@@ -79,8 +81,11 @@ func insensitiviseVal(val interface{}) interface{} {
 		// nested map: recursively insensitivise
 		insensitiviseMap(val.(map[string]interface{}))
 	case []interface{}:
-		// nested array: recursively insensitivise
-		insensitiveArray(val.([]interface{}))
+		// deprecated, drop in Viper v2
+		if !features.Revert1387 {
+			// nested array: recursively insensitivise
+			insensitiveArray(val.([]interface{}))
+		}
 	}
 	return val
 }

viper_test.go

@@ -31,6 +31,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/spf13/viper/internal/features"
 	"github.com/spf13/viper/internal/testutil"
 )
 
@@ -2672,6 +2673,22 @@ func TestSliceIndexAccess(t *testing.T) {
 	assert.Equal(t, "The Expanse", v.GetString("tv.0.title_i18n.USA"))
 	assert.Equal(t, "エクスパンス -巨獣めざめる-", v.GetString("tv.0.title_i18n.Japan"))
 
+	var expectedtitlei18n map[string]any
+
+	if features.Revert1387 {
+		expectedtitlei18n = map[string]any{
+			"USA":   "The Expanse",
+			"Japan": "エクスパンス -巨獣めざめる-",
+		}
+	} else {
+		expectedtitlei18n = map[string]any{
+			"usa":   "The Expanse",
+			"japan": "エクスパンス -巨獣めざめる-",
+		}
+	}
+
+	assert.Equal(t, expectedtitlei18n, v.GetStringMap("tv.0.title_i18n"))
+
 	// Test for index out of bounds
 	assert.Equal(t, "", v.GetString("tv.0.seasons.2.first_released"))