FIR: allow no else branch for when on Nothing(?)

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

@@ -32089,6 +32089,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                 runTest("compiler/testData/diagnostics/tests/when/whenAndLambdaWithExpectedType.kt");
             }
 
+            @Test
+            @TestMetadata("whenOnNothing.kt")
+            public void testWhenOnNothing() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/when/whenOnNothing.kt");
+            }
+
             @Test
             @TestMetadata("WhenTypeDisjunctions.kt")
             public void testWhenTypeDisjunctions() throws Exception {

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

@@ -32089,6 +32089,12 @@ public class FirOldFrontendDiagnosticsWithLightTreeTestGenerated extends Abstrac
                 runTest("compiler/testData/diagnostics/tests/when/whenAndLambdaWithExpectedType.kt");
             }
 
+            @Test
+            @TestMetadata("whenOnNothing.kt")
+            public void testWhenOnNothing() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/when/whenOnNothing.kt");
+            }
+
             @Test
             @TestMetadata("WhenTypeDisjunctions.kt")
             public void testWhenTypeDisjunctions() throws Exception {

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

@@ -10,7 +10,9 @@ import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.diagnostics.WhenMissingCase
 import org.jetbrains.kotlin.fir.FirElement
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.declarations.*
+import org.jetbrains.kotlin.fir.declarations.FirEnumEntry
+import org.jetbrains.kotlin.fir.declarations.FirRegularClass
+import org.jetbrains.kotlin.fir.declarations.getSealedClassInheritors
 import org.jetbrains.kotlin.fir.declarations.utils.collectEnumEntries
 import org.jetbrains.kotlin.fir.declarations.utils.modality
 import org.jetbrains.kotlin.fir.expressions.*
@@ -33,7 +35,8 @@ class FirWhenExhaustivenessTransformer(private val bodyResolveComponents: BodyRe
         private val exhaustivenessCheckers = listOf(
             WhenOnBooleanExhaustivenessChecker,
             WhenOnEnumExhaustivenessChecker,
-            WhenOnSealedClassExhaustivenessChecker
+            WhenOnSealedClassExhaustivenessChecker,
+            WhenOnNothingExhaustivenessChecker
         )
 
         @OptIn(ExperimentalStdlibApi::class)
@@ -389,3 +392,18 @@ private object WhenOnSealedClassExhaustivenessChecker : WhenExhaustivenessChecke
         }
     }
 }
+
+private object WhenOnNothingExhaustivenessChecker : WhenExhaustivenessChecker() {
+    override fun isApplicable(subjectType: ConeKotlinType, session: FirSession): Boolean {
+        return subjectType.isNullableNothing || subjectType.isNothing
+    }
+
+    override fun computeMissingCases(
+        whenExpression: FirWhenExpression,
+        subjectType: ConeKotlinType,
+        session: FirSession,
+        destination: MutableCollection<WhenMissingCase>
+    ) {
+        // Nothing has no branches. The null case for `Nothing?` is handled by WhenOnNullableExhaustivenessChecker
+    }
+}

compiler/testData/diagnostics/tests/when/whenOnNothing.fir.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
+
+// exhaustive
+fun test1(n: Nothing) = when (n) { }
+fun test2(n: Nothing?) = when (n) {
+    null -> {}
+}
+
+// not exhaustive
+fun test3(n: Nothing?) = <!NO_ELSE_IN_WHEN!>when<!> (n) {
+}

compiler/testData/diagnostics/tests/when/whenOnNothing.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
+
+// exhaustive
+fun test1(n: Nothing) = when (n) { }
+fun test2(n: Nothing?) = <!NO_ELSE_IN_WHEN!>when<!> (<!DEBUG_INFO_CONSTANT!>n<!>) {
+    null -> {}
+}
+
+// not exhaustive
+fun test3(n: Nothing?) = <!NO_ELSE_IN_WHEN!>when<!> (<!DEBUG_INFO_CONSTANT!>n<!>) {
+}

compiler/testData/diagnostics/tests/when/whenOnNothing.txt

@@ -0,0 +1,3 @@
+package
+
+public fun foo(/*0*/ n: kotlin.Nothing, /*1*/ nn: kotlin.Nothing?): kotlin.Unit

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -32185,6 +32185,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/when/whenAndLambdaWithExpectedType.kt");
             }
 
+            @Test
+            @TestMetadata("whenOnNothing.kt")
+            public void testWhenOnNothing() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/when/whenOnNothing.kt");
+            }
+
             @Test
             @TestMetadata("WhenTypeDisjunctions.kt")
             public void testWhenTypeDisjunctions() throws Exception {

idea/idea-frontend-fir/idea-fir-low-level-api/tests/org/jetbrains/kotlin/idea/fir/low/level/api/diagnostic/compiler/based/DiagnosisCompilerTestFE10TestdataTestGenerated.java

@@ -32089,6 +32089,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
                 runTest("compiler/testData/diagnostics/tests/when/whenAndLambdaWithExpectedType.kt");
             }
 
+            @Test
+            @TestMetadata("whenOnNothing.kt")
+            public void testWhenOnNothing() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/when/whenOnNothing.kt");
+            }
+
             @Test
             @TestMetadata("WhenTypeDisjunctions.kt")
             public void testWhenTypeDisjunctions() throws Exception {