[FIR] Implement ELSE_MISPLACED_IN_WHEN diagnostics, fix tests

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -44,6 +44,7 @@ import org.jetbrains.kotlin.psi.KtTypeParameter
 import org.jetbrains.kotlin.psi.KtTypeParameterList
 import org.jetbrains.kotlin.psi.KtTypeReference
 import org.jetbrains.kotlin.psi.KtValueArgument
+import org.jetbrains.kotlin.psi.KtWhenEntry
 import org.jetbrains.kotlin.psi.KtWhenExpression
 
 /*
@@ -1793,6 +1794,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.ELSE_MISPLACED_IN_WHEN) { firDiagnostic ->
+        ElseMisplacedInWhenImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.TYPE_PARAMETER_IS_NOT_AN_EXPRESSION) { firDiagnostic ->
         TypeParameterIsNotAnExpressionImpl(
             firSymbolBuilder.classifierBuilder.buildTypeParameterSymbol(firDiagnostic.a.fir),

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -50,6 +50,7 @@ import org.jetbrains.kotlin.psi.KtTypeParameter
 import org.jetbrains.kotlin.psi.KtTypeParameterList
 import org.jetbrains.kotlin.psi.KtTypeReference
 import org.jetbrains.kotlin.psi.KtValueArgument
+import org.jetbrains.kotlin.psi.KtWhenEntry
 import org.jetbrains.kotlin.psi.KtWhenExpression
 import org.jetbrains.kotlin.resolve.ForbiddenNamedArgumentsTarget
 
@@ -1260,6 +1261,10 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = InvalidIfAsExpression::class
     }
 
+    abstract class ElseMisplacedInWhen : KtFirDiagnostic<KtWhenEntry>() {
+        override val diagnosticClass get() = ElseMisplacedInWhen::class
+    }
+
     abstract class TypeParameterIsNotAnExpression : KtFirDiagnostic<KtSimpleNameExpression>() {
         override val diagnosticClass get() = TypeParameterIsNotAnExpression::class
         abstract val typeParameter: KtTypeParameterSymbol

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -52,6 +52,7 @@ import org.jetbrains.kotlin.psi.KtTypeParameter
 import org.jetbrains.kotlin.psi.KtTypeParameterList
 import org.jetbrains.kotlin.psi.KtTypeReference
 import org.jetbrains.kotlin.psi.KtValueArgument
+import org.jetbrains.kotlin.psi.KtWhenEntry
 import org.jetbrains.kotlin.psi.KtWhenExpression
 import org.jetbrains.kotlin.resolve.ForbiddenNamedArgumentsTarget
 
@@ -2041,6 +2042,13 @@ internal class InvalidIfAsExpressionImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class ElseMisplacedInWhenImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ElseMisplacedInWhen(), KtAbstractFirDiagnostic<KtWhenEntry> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class TypeParameterIsNotAnExpressionImpl(
     override val typeParameter: KtTypeParameterSymbol,
     firDiagnostic: FirPsiDiagnostic<*>,

idea/testData/checker/When.fir.kt

@@ -35,7 +35,7 @@ fun test() {
   val z = 1
 
   when (z) {
-    else -> 1
+    <error descr="[ELSE_MISPLACED_IN_WHEN] ">else</error> -> 1
     1 -> 2
   }