[FIR] Implement SPREAD_OF_NULLABLE

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

@@ -1243,6 +1243,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.SPREAD_OF_NULLABLE) { firDiagnostic ->
+        SpreadOfNullableImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.OVERLOAD_RESOLUTION_AMBIGUITY) { firDiagnostic ->
         OverloadResolutionAmbiguityImpl(
             firDiagnostic.a.map { firBasedSymbol ->

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

@@ -888,6 +888,10 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val name: String
     }
 
+    abstract class SpreadOfNullable : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = SpreadOfNullable::class
+    }
+
     abstract class OverloadResolutionAmbiguity : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = OverloadResolutionAmbiguity::class
         abstract val candidates: List<KtSymbol>

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

@@ -1426,6 +1426,13 @@ internal class NewInferenceNoInformationForParameterImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class SpreadOfNullableImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.SpreadOfNullable(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class OverloadResolutionAmbiguityImpl(
     override val candidates: List<KtSymbol>,
     firDiagnostic: FirPsiDiagnostic,