K2: don't report RETURN_TYPE_MISMATCH on last statement returns

#KT-53198 Fixed

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirFunctionReturnTypeMismatchChecker.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.analysis.checkers.expression
 
+import org.jetbrains.kotlin.KtFakeSourceElementKind
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
@@ -36,6 +37,13 @@ object FirFunctionReturnTypeMismatchChecker : FirReturnExpressionChecker() {
             context.session.builtinTypes.unitType.coneType
         else
             targetElement.returnTypeRef.coneType
+        if (targetElement is FirAnonymousFunction &&
+            expression.source?.kind is KtFakeSourceElementKind.ImplicitReturn.FromLastStatement &&
+            functionReturnType.isUnit
+        ) {
+            return
+        }
+
         val typeContext = context.session.typeContext
         val returnExpressionType = resultExpression.typeRef.coneType
 

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirReturnSyntaxAndLabelChecker.kt

@@ -24,7 +24,7 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
 object FirReturnSyntaxAndLabelChecker : FirReturnExpressionChecker() {
     override fun check(expression: FirReturnExpression, context: CheckerContext, reporter: DiagnosticReporter) {
         val source = expression.source
-        if (source?.kind == KtFakeSourceElementKind.ImplicitReturn) return
+        if (source?.kind is KtFakeSourceElementKind.ImplicitReturn) return
 
         val labeledElement = expression.target.labeledElement
         val targetSymbol = labeledElement.symbol

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extended/UnreachableCodeChecker.kt

@@ -40,7 +40,8 @@ object UnreachableCodeChecker : FirControlFlowChecker() {
     }
 
     private val sourceKindsToSkip = setOf(
-        KtFakeSourceElementKind.ImplicitReturn,
+        KtFakeSourceElementKind.ImplicitReturn.FromExpressionBody,
+        KtFakeSourceElementKind.ImplicitReturn.FromLastStatement,
         KtFakeSourceElementKind.DesugaredForLoop
     )