FIR checker: report nullable/non-class LHS of class literals

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

@@ -679,6 +679,18 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.CLASS_LITERAL_LHS_NOT_A_CLASS) { firDiagnostic ->
+        ClassLiteralLhsNotAClassImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.NULLABLE_TYPE_IN_CLASS_LITERAL_LHS) { firDiagnostic ->
+        NullableTypeInClassLiteralLhsImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.NOTHING_TO_OVERRIDE) { firDiagnostic ->
         NothingToOverrideImpl(
             firSymbolBuilder.buildSymbol(firDiagnostic.a as FirDeclaration),

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

@@ -483,6 +483,14 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = InnerClassOfGenericThrowableSubclass::class
     }
 
+    abstract class ClassLiteralLhsNotAClass : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = ClassLiteralLhsNotAClass::class
+    }
+
+    abstract class NullableTypeInClassLiteralLhs : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = NullableTypeInClassLiteralLhs::class
+    }
+
     abstract class NothingToOverride : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = NothingToOverride::class
         abstract val declaration: KtSymbol

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

@@ -775,6 +775,20 @@ internal class InnerClassOfGenericThrowableSubclassImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class ClassLiteralLhsNotAClassImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ClassLiteralLhsNotAClass(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class NullableTypeInClassLiteralLhsImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.NullableTypeInClassLiteralLhs(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class NothingToOverrideImpl(
     override val declaration: KtSymbol,
     firDiagnostic: FirPsiDiagnostic<*>,