[FIR] Fix building CFG and resolving when expression without branches

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirDataFlowAnalyzer.kt

@@ -353,15 +353,20 @@ class FirDataFlowAnalyzer(transformer: FirBodyResolveTransformer) : BodyResolveC
     fun exitWhenExpression(whenExpression: FirWhenExpression) {
         val (whenExitNode, syntheticElseNode) = graphBuilder.exitWhenExpression(whenExpression)
         if (syntheticElseNode != null) {
-            val previousConditionExitNode = syntheticElseNode.previousNodes.single() as WhenBranchConditionExitNode
             syntheticElseNode.mergeIncomingFlow()
-            syntheticElseNode.flow = logicSystem.approveFactsInsideFlow(
-                variablesForWhenConditions.remove(previousConditionExitNode)!!,
-                EqFalse,
-                syntheticElseNode.flow,
-                shouldForkFlow = true,
-                shouldRemoveSynthetics = true
-            )
+            val previousConditionExitNode = syntheticElseNode.previousNodes.single() as? WhenBranchConditionExitNode
+            // previous node for syntheticElseNode can be not WhenBranchConditionExitNode in case of `when` without any branches
+            // in that case there will be when enter or subject access node
+            if (previousConditionExitNode != null) {
+                syntheticElseNode.flow = logicSystem.approveFactsInsideFlow(
+                    variablesForWhenConditions.remove(previousConditionExitNode)!!,
+                    EqFalse,
+                    syntheticElseNode.flow,
+                    shouldForkFlow = true,
+                    shouldRemoveSynthetics = true
+                )
+            }
+
         }
         val previousFlows = whenExitNode.alivePreviousNodes.map { it.flow }
         val flow = logicSystem.joinFlow(previousFlows)

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraphBuilder.kt

@@ -225,7 +225,6 @@ class ControlFlowGraphBuilder {
         // exit from last condition node still on stack
         // we should remove it
         val lastWhenConditionExit = lastNodes.pop()
-        assert(lastWhenConditionExit is WhenBranchConditionExitNode)
         val syntheticElseBranchNode = if (!whenExpression.isExhaustive) {
             createWhenSyntheticElseBranchNode(whenExpression).apply {
                 addEdge(lastWhenConditionExit, this)

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirBodyResolveTransformer.kt

@@ -532,20 +532,25 @@ open class FirBodyResolveTransformer(
             }
             @Suppress("NAME_SHADOWING")
             var whenExpression = whenExpression.transformSubject(this, noExpectedType)
-            if (whenExpression.isOneBranch()) {
-                whenExpression = whenExpression.transformBranches(this, noExpectedType)
-                whenExpression.resultType = whenExpression.branches.first().result.resultType
-            } else {
-                whenExpression = whenExpression.transformBranches(this, null)
 
-                whenExpression = syntheticCallGenerator.generateCalleeForWhenExpression(whenExpression) ?: run {
-                    dataFlowAnalyzer.exitWhenExpression(whenExpression)
-                    whenExpression.resultType = FirErrorTypeRefImpl(null, "")
-                    return@with whenExpression.compose()
+            when {
+                whenExpression.branches.isEmpty() -> {}
+                whenExpression.isOneBranch() -> {
+                    whenExpression = whenExpression.transformBranches(this, noExpectedType)
+                    whenExpression.resultType = whenExpression.branches.first().result.resultType
                 }
+                else -> {
+                    whenExpression = whenExpression.transformBranches(this, null)
 
-                val expectedTypeRef = data as FirTypeRef?
-                whenExpression = callCompleter.completeCall(whenExpression, expectedTypeRef)
+                    whenExpression = syntheticCallGenerator.generateCalleeForWhenExpression(whenExpression) ?: run {
+                        dataFlowAnalyzer.exitWhenExpression(whenExpression)
+                        whenExpression.resultType = FirErrorTypeRefImpl(null, "")
+                        return@with whenExpression.compose()
+                    }
+
+                    val expectedTypeRef = data as FirTypeRef?
+                    whenExpression = callCompleter.completeCall(whenExpression, expectedTypeRef)
+                }
             }
             whenExpression = whenExpression.transformSingle(whenExhaustivenessTransformer, null)
             dataFlowAnalyzer.exitWhenExpression(whenExpression)

compiler/fir/resolve/testData/resolve/cfg/emptyWhen.dot

@@ -0,0 +1,84 @@
+digraph emptyWhen_kt {
+  graph [splines=ortho nodesep=3]
+  node [shape=box penwidth=2]
+  edge [penwidth=2]
+
+  subgraph cluster_0 {
+    color=red
+    0 [label="Enter function test_1" style="filled" fillcolor=red];
+    subgraph cluster_1 {
+      color=blue
+      1 [label="Enter block"];
+      subgraph cluster_2 {
+        color=blue
+        2 [label="Enter when"];
+        3 [label="Synthetic else branch"];
+        4 [label="Exit when"];
+      }
+      5 [label="Exit block"];
+    }
+    6 [label="Exit function test_1" style="filled" fillcolor=red];
+  }
+
+  0 -> {1};
+  1 -> {2};
+  2 -> {3};
+  3 -> {4};
+  4 -> {5};
+  5 -> {6};
+
+  subgraph cluster_3 {
+    color=red
+    7 [label="Enter function test_2" style="filled" fillcolor=red];
+    subgraph cluster_4 {
+      color=blue
+      8 [label="Enter block"];
+      subgraph cluster_5 {
+        color=blue
+        9 [label="Enter when"];
+        10 [label="Access variable R|<local>/x|"];
+        11 [label="Synthetic else branch"];
+        12 [label="Exit when"];
+      }
+      13 [label="Exit block"];
+    }
+    14 [label="Exit function test_2" style="filled" fillcolor=red];
+  }
+
+  7 -> {8};
+  8 -> {9};
+  9 -> {10};
+  10 -> {11};
+  11 -> {12};
+  12 -> {13};
+  13 -> {14};
+
+  subgraph cluster_6 {
+    color=red
+    15 [label="Enter function test_3" style="filled" fillcolor=red];
+    subgraph cluster_7 {
+      color=blue
+      16 [label="Enter block"];
+      subgraph cluster_8 {
+        color=blue
+        17 [label="Enter when"];
+        18 [label="Access variable R|<local>/x|"];
+        19 [label="Variable declaration: lval y: R|kotlin/Int|"];
+        20 [label="Synthetic else branch"];
+        21 [label="Exit when"];
+      }
+      22 [label="Exit block"];
+    }
+    23 [label="Exit function test_3" style="filled" fillcolor=red];
+  }
+
+  15 -> {16};
+  16 -> {17};
+  17 -> {18};
+  18 -> {19};
+  19 -> {20};
+  20 -> {21};
+  21 -> {22};
+  22 -> {23};
+
+}

compiler/fir/resolve/testData/resolve/cfg/emptyWhen.kt

@@ -0,0 +1,11 @@
+fun test_1() {
+    when {}
+}
+
+fun test_2(x: Int) {
+    when (x) {}
+}
+
+fun test_3(x: Int) {
+    when (val y = x) {}
+}

compiler/fir/resolve/testData/resolve/cfg/emptyWhen.txt

@@ -0,0 +1,16 @@
+FILE: emptyWhen.kt
+    public final fun test_1(): R|kotlin/Unit| {
+        when () {
+        }
+
+    }
+    public final fun test_2(x: R|kotlin/Int|): R|kotlin/Unit| {
+        when (R|<local>/x|) {
+        }
+
+    }
+    public final fun test_3(x: R|kotlin/Int|): R|kotlin/Unit| {
+        when (lval y: R|kotlin/Int| = R|<local>/x|) {
+        }
+
+    }

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirCfgBuildingTestGenerated.java

@@ -46,6 +46,11 @@ public class FirCfgBuildingTestGenerated extends AbstractFirCfgBuildingTest {
             runTest("compiler/fir/resolve/testData/resolve/cfg/complex.kt");
         }
 
+        @TestMetadata("emptyWhen.kt")
+        public void testEmptyWhen() throws Exception {
+            runTest("compiler/fir/resolve/testData/resolve/cfg/emptyWhen.kt");
+        }
+
         @TestMetadata("initBlock.kt")
         public void testInitBlock() throws Exception {
             runTest("compiler/fir/resolve/testData/resolve/cfg/initBlock.kt");