[FIR] Fix false positive NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY

compiler/fir/analysis-tests/testData/resolve/elvisReturnSimple.dot

@@ -0,0 +1,98 @@
+digraph elvisReturnSimple_kt {
+    graph [nodesep=3]
+    node [shape=box penwidth=2]
+    edge [penwidth=2]
+
+    subgraph cluster_0 {
+        color=red
+        0 [label="Enter function stringReturnInLeftLen" style="filled" fillcolor=red];
+        subgraph cluster_1 {
+            color=blue
+            1 [label="Enter block"];
+            subgraph cluster_2 {
+                color=blue
+                2 [label="Enter when"];
+                subgraph cluster_3 {
+                    color=blue
+                    3 [label="Enter when branch condition "];
+                    4 [label="Access variable R|<local>/s|"];
+                    5 [label="Const: Null(null)"];
+                    6 [label="Equality operator !="];
+                    7 [label="Exit when branch condition"];
+                }
+                subgraph cluster_4 {
+                    color=blue
+                    8 [label="Enter when branch condition else"];
+                    9 [label="Exit when branch condition"];
+                }
+                10 [label="Enter when branch result"];
+                subgraph cluster_5 {
+                    color=blue
+                    11 [label="Enter block"];
+                    12 [label="Const: Null(null)"];
+                    13 [label="Exit block"];
+                }
+                14 [label="Exit when branch result"];
+                15 [label="Enter when branch result"];
+                subgraph cluster_6 {
+                    color=blue
+                    16 [label="Enter block"];
+                    17 [label="Access variable R|<local>/s|"];
+                    18 [label="Access variable R|kotlin/String.length|"];
+                    19 [label="Jump: ^stringReturnInLeftLen R|<local>/s|.R|kotlin/String.length|"];
+                    20 [label="Stub" style="filled" fillcolor=gray];
+                    21 [label="Exit block" style="filled" fillcolor=gray];
+                }
+                22 [label="Exit when branch result" style="filled" fillcolor=gray];
+                23 [label="Exit when"];
+            }
+            24 [label="Exit lhs of ?:"];
+            25 [label="Enter rhs of ?:"];
+            26 [label="Const: Int(0)"];
+            27 [label="Jump: ^stringReturnInLeftLen Int(0)"];
+            28 [label="Stub" style="filled" fillcolor=gray];
+            29 [label="Lhs of ?: is not null" style="filled" fillcolor=gray];
+            30 [label="Exit ?:" style="filled" fillcolor=gray];
+            31 [label="Variable declaration: lval s1: R|kotlin/Nothing|" style="filled" fillcolor=gray];
+            32 [label="Exit block" style="filled" fillcolor=gray];
+        }
+        33 [label="Exit function stringReturnInLeftLen" style="filled" fillcolor=red];
+    }
+    0 -> {1};
+    1 -> {2};
+    2 -> {3};
+    3 -> {4};
+    4 -> {5};
+    5 -> {6};
+    6 -> {7};
+    7 -> {15 8};
+    8 -> {9};
+    9 -> {10};
+    10 -> {11};
+    11 -> {12};
+    12 -> {13};
+    13 -> {14};
+    14 -> {23};
+    15 -> {16};
+    16 -> {17};
+    17 -> {18};
+    18 -> {19};
+    19 -> {33};
+    19 -> {20} [style=dotted];
+    20 -> {21} [style=dotted];
+    21 -> {22} [style=dotted];
+    22 -> {23} [style=dotted];
+    23 -> {24};
+    24 -> {25};
+    24 -> {29} [style=dotted];
+    25 -> {26};
+    26 -> {27};
+    27 -> {33};
+    27 -> {28} [style=dotted];
+    28 -> {30} [style=dotted];
+    29 -> {30} [style=dotted];
+    30 -> {31} [style=dotted];
+    31 -> {32} [style=dotted];
+    32 -> {33} [style=dotted];
+
+}

compiler/fir/analysis-tests/testData/resolve/elvisReturnSimple.fir.txt

@@ -0,0 +1,12 @@
+FILE: elvisReturnSimple.kt
+    public final fun stringReturnInLeftLen(s: R|kotlin/String?|): R|kotlin/Int| {
+        lval s1: R|kotlin/Nothing| = when () {
+            !=(R|<local>/s|, Null(null)) ->  {
+                ^stringReturnInLeftLen R|<local>/s|.R|kotlin/String.length|
+            }
+            else ->  {
+                Null(null)
+            }
+        }
+         ?: ^stringReturnInLeftLen Int(0)
+    }

compiler/fir/analysis-tests/testData/resolve/elvisReturnSimple.kt

@@ -0,0 +1,6 @@
+// DUMP_CFG
+
+fun stringReturnInLeftLen(s : String?) : Int {
+    // this shouldn't trigger NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY
+    val s1 = (if (s != null) { return s.length } else { null }) ?: return 0
+}

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

@@ -19,7 +19,7 @@ import org.jetbrains.kotlin.fir.resolve.dfa.*
 import org.jetbrains.kotlin.fir.resolve.transformers.body.resolve.resultType
 import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
-import org.jetbrains.kotlin.fir.types.isNothing
+import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.util.ListMultimap
 import org.jetbrains.kotlin.fir.util.listMultimapOf
 import org.jetbrains.kotlin.fir.visitors.FirDefaultVisitor
@@ -1317,13 +1317,22 @@ class ControlFlowGraphBuilder {
             exitElvisExpressionNodes.push(it)
         }
 
+        val typedFir = lastNodes.topOrNull()?.fir as? FirExpression
+        val type = typedFir?.typeRef?.coneTypeSafe<ConeKotlinType>()
+
         val lhsExitNode = createElvisLhsExitNode(elvisExpression).also {
             popAndAddEdge(it)
         }
 
         val lhsIsNotNullNode = createElvisLhsIsNotNullNode(elvisExpression).also {
-            addEdge(lhsExitNode, it)
+            val preferredKind = if (type?.isNullableNothing == true) {
-            addEdge(it, exitNode)
+                EdgeKind.DeadForward
+            } else {
+                EdgeKind.Forward
+            }
+
+            addEdge(lhsExitNode, it, preferredKind = preferredKind)
+            addEdge(it, exitNode, propagateDeadness = false)
         }
 
         val rhsEnterNode = elvisRhsEnterNodes.pop().also {