FIR checker: COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT

FIR simply ignores anything after the first comma if the when expression
doesn't have a subject. Hence, the checker has to rely on PSI structure
instead.

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

@@ -2803,6 +2803,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT) { firDiagnostic ->
+        CommaInWhenConditionWithoutArgumentImpl(
+            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

@@ -1958,6 +1958,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val illegalReason: String
     }
 
+    abstract class CommaInWhenConditionWithoutArgument : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = CommaInWhenConditionWithoutArgument::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

@@ -3156,6 +3156,13 @@ internal class IllegalDeclarationInWhenSubjectImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class CommaInWhenConditionWithoutArgumentImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.CommaInWhenConditionWithoutArgument(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class TypeParameterIsNotAnExpressionImpl(
     override val typeParameter: KtTypeParameterSymbol,
     firDiagnostic: FirPsiDiagnostic,