FIR checker: make FirMemberFunctionsChecker a function checker

There is no reason why this checker needs to be a class checker.

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt

@@ -55,6 +55,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
             FirOperatorModifierChecker,
             FirTailrecFunctionChecker,
             FirTopLevelFunctionsChecker,
+            FirMemberFunctionsChecker,
         )
 
     override val propertyCheckers: Set<FirPropertyChecker>
@@ -88,7 +89,6 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
             FirOpenMemberChecker,
             FirClassVarianceChecker,
             FirSealedSupertypeChecker,
-            FirMemberFunctionsChecker,
             FirMemberPropertiesChecker,
             FirImplementationMismatchChecker,
             FirTypeParametersInObjectChecker,

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

@@ -7,26 +7,24 @@ package org.jetbrains.kotlin.fir.analysis.checkers.declaration
 
 import org.jetbrains.kotlin.KtFakeSourceElementKind
 import org.jetbrains.kotlin.descriptors.Visibilities
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.analysis.checkers.contains
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.analysis.checkers.getModifierList
-import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
-import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.declarations.FirClass
 import org.jetbrains.kotlin.fir.declarations.FirRegularClass
 import org.jetbrains.kotlin.fir.declarations.FirSimpleFunction
 import org.jetbrains.kotlin.fir.declarations.utils.*
 import org.jetbrains.kotlin.lexer.KtTokens
+import org.jetbrains.kotlin.utils.addToStdlib.lastIsInstanceOrNull
 
 // See old FE's [DeclarationsChecker]
-object FirMemberFunctionsChecker : FirClassChecker() {
+object FirMemberFunctionsChecker : FirSimpleFunctionChecker() {
-    override fun check(declaration: FirClass, context: CheckerContext, reporter: DiagnosticReporter) {
+    override fun check(declaration: FirSimpleFunction, context: CheckerContext, reporter: DiagnosticReporter) {
-        for (member in declaration.declarations) {
+        val containingDeclaration = context.containingDeclarations.lastIsInstanceOrNull<FirClass>() ?: return
-            if (member is FirSimpleFunction) {
+        checkFunction(containingDeclaration, declaration, context, reporter)
-                checkFunction(declaration, member, context, reporter)
-            }
-        }
     }
 
     private fun checkFunction(