[FIR] Add REDUNDANT_NULLABLE diagnostics

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

@@ -1891,6 +1891,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.REDUNDANT_NULLABLE) { firDiagnostic ->
+        RedundantNullableImpl(
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED) { firDiagnostic ->
         ExtensionInClassReferenceNotAllowedImpl(
             firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.a.fir),

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

@@ -1348,6 +1348,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val isCastToNotNull: Boolean
     }
 
+    abstract class RedundantNullable : KtFirDiagnostic<KtTypeReference>() {
+        override val diagnosticClass get() = RedundantNullable::class
+    }
+
     abstract class ExtensionInClassReferenceNotAllowed : KtFirDiagnostic<KtExpression>() {
         override val diagnosticClass get() = ExtensionInClassReferenceNotAllowed::class
         abstract val referencedDeclaration: KtCallableSymbol

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

@@ -1617,6 +1617,11 @@ internal class SmartcastImpossibleImpl(
     override val token: ValidityToken,
 ) : KtFirDiagnostic.SmartcastImpossible(), KtAbstractFirDiagnostic<KtExpression>
 
+internal class RedundantNullableImpl(
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.RedundantNullable(), KtAbstractFirDiagnostic<KtTypeReference>
+
 internal class ExtensionInClassReferenceNotAllowedImpl(
     override val referencedDeclaration: KtCallableSymbol,
     override val firDiagnostic: KtPsiDiagnostic,

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostics/AbstractFirIdeDiagnosticsCollector.kt

@@ -90,7 +90,7 @@ private object CheckersFactory {
     }
 
     private fun createTypeCheckers(useExtendedCheckers: Boolean): TypeCheckers? =
-        if (useExtendedCheckers) null else CommonTypeCheckers
+        if (useExtendedCheckers) ExtendedTypeCheckers else CommonTypeCheckers
 
 
     @OptIn(ExperimentalStdlibApi::class)