[FIR] Unwrap substitution overrides for multiple default values

Otherwise, duplicate diagnostics may be reported on
subtypes of the type that actually declares the
problematic override.

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirMultipleDefaultsInheritedFromSupertypesChecker.kt

@@ -20,16 +20,19 @@ import org.jetbrains.kotlin.fir.isSubstitutionOverride
 import org.jetbrains.kotlin.fir.scopes.processAllFunctions
 import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirValueParameterSymbol
+import org.jetbrains.kotlin.fir.unwrapSubstitutionOverrides
 import org.jetbrains.kotlin.name.Name
 
 object FirMultipleDefaultsInheritedFromSupertypesChecker : FirRegularClassChecker() {
     override fun check(declaration: FirRegularClass, context: CheckerContext, reporter: DiagnosticReporter) {
         declaration.unsubstitutedScope(context).processAllFunctions {
-            if (it.containingClassLookupTag() != declaration.symbol.toLookupTag()) {
+            val originalIfSubstitutionOverride = it.unwrapSubstitutionOverrides()
+
+            if (originalIfSubstitutionOverride.containingClassLookupTag() != declaration.symbol.toLookupTag()) {
                 return@processAllFunctions
             }
 
-            checkFunction(declaration, it, context, reporter)
+            checkFunction(declaration, originalIfSubstitutionOverride, context, reporter)
         }
     }
 

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndSubstitutionOverrides.fir.kt

@@ -1,15 +0,0 @@
-interface A<T> {
-    fun foo(x: Int = 42): Int
-}
-
-open class B<K> {
-    fun foo(x: Int = 239) = x
-}
-
-interface C<E> {
-    fun foo(x: Int): Int
-}
-
-open <!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE!>class Z<!><R> : A<R>, B<R>(), C<R>
-
-<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION_WARNING!>class N<!><W> : Z<W>()

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndSubstitutionOverrides.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface A<T> {
     fun foo(x: Int = 42): Int
 }