[FIR] Fix detecting that if in then branch of outer if used as expression

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

@@ -1333,6 +1333,11 @@ public class LazyBodyIsNotTouchedTilContractsPhaseTestGenerated extends Abstract
                 runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveness_sealedSubClass.kt");
             }
 
+            @TestMetadata("nestedIfInLambda.kt")
+            public void testNestedIfInLambda() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.kt");
+            }
+
             @TestMetadata("nonExhaustiveWhenWithCoercionToUnit.kt")
             public void testNonExhaustiveWhenWithCoercionToUnit() throws Exception {
                 runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nonExhaustiveWhenWithCoercionToUnit.kt");

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.fir.txt

@@ -0,0 +1,20 @@
+FILE: nestedIfInLambda.kt
+    public final fun run(block: R|() -> kotlin/Unit|): R|kotlin/Unit| {
+    }
+    public final fun test(b1: R|kotlin/Boolean|, b2: R|kotlin/Boolean|): R|kotlin/Unit| {
+        lvar result: R|kotlin/Boolean| = Boolean(false)
+        R|/run|(<L> = run@fun <anonymous>(): R|kotlin/Unit| {
+            when () {
+                R|<local>/b1| ->  {
+                    when () {
+                        R|<local>/b2| ->  {
+                            R|<local>/result| = Boolean(true)
+                        }
+                    }
+
+                }
+            }
+
+        }
+        )
+    }

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.kt

@@ -0,0 +1,10 @@
+fun run(block: () -> Unit) {}
+
+fun test(b1: Boolean, b2: Boolean) {
+    var result = false
+    run {
+        if (b1)
+            if (b2)
+                result = true
+    }
+}

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirDiagnosticTestGenerated.java

@@ -1524,6 +1524,12 @@ public class FirDiagnosticTestGenerated extends AbstractFirDiagnosticTest {
                     runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveness_sealedSubClass.kt");
                 }
 
+                @Test
+                @TestMetadata("nestedIfInLambda.kt")
+                public void testNestedIfInLambda() throws Exception {
+                    runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("nonExhaustiveWhenWithCoercionToUnit.kt")
                 public void testNonExhaustiveWhenWithCoercionToUnit() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirDiagnosticsWithLightTreeTestGenerated.java

@@ -1539,6 +1539,12 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
                     runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveness_sealedSubClass.kt");
                 }
 
+                @Test
+                @TestMetadata("nestedIfInLambda.kt")
+                public void testNestedIfInLambda() throws Exception {
+                    runTest("compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("nonExhaustiveWhenWithCoercionToUnit.kt")
                 public void testNonExhaustiveWhenWithCoercionToUnit() throws Exception {

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt

@@ -1111,7 +1111,7 @@ class ExpressionsConverter(
             }
             val parentTokenType = parent.tokenType
             if (parentTokenType == BLOCK) return false
-            if (parentTokenType == ELSE || parentTokenType == WHEN_ENTRY) {
+            if (parentTokenType == THEN || parentTokenType == ELSE || parentTokenType == WHEN_ENTRY) {
                 return parent.getParent()?.usedAsExpression ?: true
             }
             if (parentTokenType != BODY) return true

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -1631,7 +1631,7 @@ class RawFirBuilder(
                 }
                 if (parent is KtBlockExpression) return false
                 when (parent.elementType) {
-                    KtNodeTypes.ELSE, KtNodeTypes.WHEN_ENTRY -> {
+                    KtNodeTypes.THEN, KtNodeTypes.ELSE, KtNodeTypes.WHEN_ENTRY -> {
                         return (parent.parent as? KtExpression)?.usedAsExpression ?: true
                     }
                 }