Ignore properties starting with _ in backing-property-naming rule (#2753)

Name of an overridden property can only be changed by changing the name of the base property.

Closes #2748

Author: paul-dingemans

ktlint-ruleset-standard/src/main/kotlin/com/pinterest/ktlint/ruleset/standard/rules/BackingPropertyNamingRule.kt

@@ -4,6 +4,7 @@ import com.pinterest.ktlint.rule.engine.core.api.AutocorrectDecision
 import com.pinterest.ktlint.rule.engine.core.api.ElementType.FUN
 import com.pinterest.ktlint.rule.engine.core.api.ElementType.IDENTIFIER
 import com.pinterest.ktlint.rule.engine.core.api.ElementType.INTERNAL_KEYWORD
+import com.pinterest.ktlint.rule.engine.core.api.ElementType.OVERRIDE_KEYWORD
 import com.pinterest.ktlint.rule.engine.core.api.ElementType.PRIVATE_KEYWORD
 import com.pinterest.ktlint.rule.engine.core.api.ElementType.PROPERTY
 import com.pinterest.ktlint.rule.engine.core.api.ElementType.PROTECTED_KEYWORD
@@ -53,7 +54,10 @@ public class BackingPropertyNamingRule :
         property
             .findChildByType(IDENTIFIER)
             ?.takeIf { it.text.startsWith("_") }
-            ?.let { identifier ->
+            ?.takeUnless {
+                // Do not report overridden properties as they can only be changed by changing the base property
+                it.treeParent.hasModifier(OVERRIDE_KEYWORD)
+            }?.let { identifier ->
                 visitBackingProperty(identifier, emit)
             }
     }

ktlint-ruleset-standard/src/test/kotlin/com/pinterest/ktlint/ruleset/standard/rules/BackingPropertyNamingRuleTest.kt

@@ -345,4 +345,18 @@ class BackingPropertyNamingRuleTest {
             """.trimIndent()
         backingPropertyNamingRuleAssertThat(code).hasNoLintViolations()
     }
+
+    @Test
+    fun `Issue 2748 - Given an override property with name starting with '_' then do not report a violation`() {
+        val code =
+            """
+            // The property "__foo" in example below can be defined in an external dependency, which can not be changed. Even in case it is
+            // a (internal) dependency that can be changed, the violation should only be reported at the base property, but on the overrides
+            val fooBar =
+                object : FooBar {
+                    override val __foo = "foo"
+                }
+            """.trimIndent()
+        backingPropertyNamingRuleAssertThat(code).hasNoLintViolations()
+    }
 }