FIR checker: make unused checker handle invoke properly

#KT-43688 Fixed

compiler/fir/analysis-tests/testData/extendedCheckers/unused/invoke.kt

@@ -0,0 +1,5 @@
+fun foo(): Int {
+    val x = fun() = 4
+    val y = fun() = 2
+    return 10 * x() + y()
+}

compiler/fir/analysis-tests/testData/extendedCheckers/unused/invoke.txt

@@ -0,0 +1,12 @@
+FILE: invoke.kt
+    public final fun foo(): R|kotlin/Int| {
+        lval x: R|() -> kotlin/Int| = fun <anonymous>(): R|kotlin/Int| {
+            ^ Int(4)
+        }
+
+        lval y: R|() -> kotlin/Int| = fun <anonymous>(): R|kotlin/Int| {
+            ^ Int(2)
+        }
+
+        ^foo Int(10).R|kotlin/Int.times|(R|<local>/x|.R|SubstitutionOverride<kotlin/Function0.invoke: R|kotlin/Int|>|()).R|kotlin/Int.plus|(R|<local>/y|.R|SubstitutionOverride<kotlin/Function0.invoke: R|kotlin/Int|>|())
+    }

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/fir/ExtendedFirDiagnosticsTestGenerated.java

@@ -330,6 +330,11 @@ public class ExtendedFirDiagnosticsTestGenerated extends AbstractExtendedFirDiag
             runTest("compiler/fir/analysis-tests/testData/extendedCheckers/unused/classProperty.kt");
         }
 
+        @TestMetadata("invoke.kt")
+        public void testInvoke() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/extendedCheckers/unused/invoke.kt");
+        }
+
         @TestMetadata("lambda.kt")
         public void testLambda() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/extendedCheckers/unused/lambda.kt");

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/fir/ExtendedFirWithLightTreeDiagnosticsTestGenerated.java

@@ -330,6 +330,11 @@ public class ExtendedFirWithLightTreeDiagnosticsTestGenerated extends AbstractEx
             runTest("compiler/fir/analysis-tests/testData/extendedCheckers/unused/classProperty.kt");
         }
 
+        @TestMetadata("invoke.kt")
+        public void testInvoke() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/extendedCheckers/unused/invoke.kt");
+        }
+
         @TestMetadata("lambda.kt")
         public void testLambda() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/extendedCheckers/unused/lambda.kt");

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

@@ -10,6 +10,7 @@ import kotlinx.collections.immutable.persistentMapOf
 import org.jetbrains.kotlin.KtNodeTypes
 import org.jetbrains.kotlin.fir.FirAnnotationContainer
 import org.jetbrains.kotlin.fir.FirFakeSourceElementKind
+import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.FirSymbolOwner
 import org.jetbrains.kotlin.fir.analysis.cfa.*
 import org.jetbrains.kotlin.fir.analysis.checkers.cfa.FirControlFlowChecker
@@ -23,7 +24,10 @@ import org.jetbrains.kotlin.fir.expressions.FirFunctionCall
 import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccess
 import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
 import org.jetbrains.kotlin.fir.resolve.dfa.cfg.*
+import org.jetbrains.kotlin.fir.resolve.inference.isFunctionalType
+import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirPropertySymbol
+import org.jetbrains.kotlin.fir.types.coneType
 
 object UnusedChecker : FirControlFlowChecker() {
     override fun analyze(graph: ControlFlowGraph, reporter: DiagnosticReporter, checkerContext: CheckerContext) {
@@ -34,7 +38,7 @@ object UnusedChecker : FirControlFlowChecker() {
         val properties = LocalPropertyCollector.collect(graph)
         if (properties.isEmpty()) return
 
-        val data = ValueWritesWithoutReading(properties).getData(graph)
+        val data = ValueWritesWithoutReading(checkerContext.session, properties).getData(graph)
         graph.traverse(TraverseDirection.Backward, CfaVisitor(data, reporter))
     }
 
@@ -151,6 +155,7 @@ object UnusedChecker : FirControlFlowChecker() {
     }
 
     private class ValueWritesWithoutReading(
+        private val session: FirSession,
         private val localProperties: Set<FirPropertySymbol>
     ) : ControlFlowGraphVisitor<PathAwareVariableStatusInfo, Collection<Pair<EdgeLabel, PathAwareVariableStatusInfo>>>() {
         fun getData(graph: ControlFlowGraph): Map<CFGNode<*>, PathAwareVariableStatusInfo> {
@@ -265,6 +270,23 @@ object UnusedChecker : FirControlFlowChecker() {
             return update(dataForNode, *symbols) { status }
         }
 
+        override fun visitFunctionCallNode(
+            node: FunctionCallNode,
+            data: Collection<Pair<EdgeLabel, PathAwareVariableStatusInfo>>
+        ): PathAwareVariableStatusInfo {
+            val dataForNode = visitNode(node, data)
+            val reference = node.fir.calleeReference as? FirResolvedNamedReference ?: return dataForNode
+            val functionSymbol = reference.resolvedSymbol as? FirFunctionSymbol<*> ?: return dataForNode
+            val symbol = if (functionSymbol.callableId.callableName.identifier == "invoke") {
+                localProperties.find { it.fir.name == reference.name && it.fir.returnTypeRef.coneType.isFunctionalType(session) }
+            } else null
+            symbol ?: return dataForNode
+
+            val status = VariableStatus.READ
+            status.isRead = true
+            return update(dataForNode, symbol) { status }
+        }
+
         private fun update(
             pathAwareInfo: PathAwareVariableStatusInfo,
             vararg symbols: FirPropertySymbol,