FIR checker: ILLEGAL_DECLARATION_IN_WHEN_SUBJECT

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

@@ -2379,7 +2379,7 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
     }
     add(FirErrors.EXPECTED_CONDITION) { firDiagnostic ->
         ExpectedConditionImpl(
-            firDiagnostic as FirPsiDiagnostic<*>,
+            firDiagnostic as FirPsiDiagnostic,
             token,
         )
     }
@@ -2404,6 +2404,13 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.ILLEGAL_DECLARATION_IN_WHEN_SUBJECT) { firDiagnostic ->
+        IllegalDeclarationInWhenSubjectImpl(
+            firDiagnostic.a,
+            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

@@ -1693,6 +1693,11 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ElseMisplacedInWhen::class
     }
 
+    abstract class IllegalDeclarationInWhenSubject : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = IllegalDeclarationInWhenSubject::class
+        abstract val illegalReason: String
+    }
+
     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

@@ -2710,10 +2710,10 @@ internal class UselessIsCheckImpl(
 }
 
 internal class ExpectedConditionImpl(
-    firDiagnostic: FirPsiDiagnostic<*>,
+    firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,
 ) : KtFirDiagnostic.ExpectedCondition(), KtAbstractFirDiagnostic<KtWhenCondition> {
-    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
 internal class NoElseInWhenImpl(
@@ -2738,6 +2738,14 @@ internal class ElseMisplacedInWhenImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class IllegalDeclarationInWhenSubjectImpl(
+    override val illegalReason: String,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.IllegalDeclarationInWhenSubject(), KtAbstractFirDiagnostic<KtElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class TypeParameterIsNotAnExpressionImpl(
     override val typeParameter: KtTypeParameterSymbol,
     firDiagnostic: FirPsiDiagnostic,