[FE] Introduce warnings on possible empty intersection types, and improve errors reporting in general

^KT-52361 Fixed

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt

@@ -5,6 +5,8 @@
 
 package org.jetbrains.kotlin.fir.types
 
+import org.jetbrains.kotlin.descriptors.Modality
+import org.jetbrains.kotlin.fir.declarations.utils.modality
 import org.jetbrains.kotlin.fir.diagnostics.ConeIntermediateDiagnostic
 import org.jetbrains.kotlin.fir.isPrimitiveNumberOrUnsignedNumberType
 import org.jetbrains.kotlin.fir.resolve.createSubstitutionForSupertype
@@ -17,6 +19,8 @@ import org.jetbrains.kotlin.fir.resolve.substitution.NoSubstitutor
 import org.jetbrains.kotlin.fir.resolve.substitution.createTypeSubstitutorByTypeConstructor
 import org.jetbrains.kotlin.fir.symbols.ConeClassLikeLookupTag
 import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterLookupTag
+import org.jetbrains.kotlin.fir.symbols.impl.FirAnonymousObjectSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
 import org.jetbrains.kotlin.fir.types.impl.ConeClassLikeTypeImpl
 import org.jetbrains.kotlin.fir.types.impl.ConeTypeParameterTypeImpl
 import org.jetbrains.kotlin.name.StandardClassIds
@@ -299,6 +303,13 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext, ConeTypeCo
         return attributes.noInfer != null
     }
 
+    override fun TypeConstructorMarker.isFinalClassConstructor(): Boolean {
+        val symbol = toClassLikeSymbol() ?: return false
+        if (symbol is FirAnonymousObjectSymbol) return true
+        val classSymbol = symbol as? FirRegularClassSymbol ?: return false
+        return classSymbol.fir.modality == Modality.FINAL
+    }
+
     override fun TypeVariableMarker.freshTypeConstructor(): TypeConstructorMarker {
         require(this is ConeTypeVariable)
         return this.typeConstructor

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeTypeContext.kt

@@ -277,7 +277,7 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
         }
     }
 
-    private fun TypeConstructorMarker.toClassLikeSymbol(): FirClassLikeSymbol<*>? = (this as? ConeClassLikeLookupTag)?.toSymbol(session)
+    fun TypeConstructorMarker.toClassLikeSymbol(): FirClassLikeSymbol<*>? = (this as? ConeClassLikeLookupTag)?.toSymbol(session)
 
     override fun TypeConstructorMarker.supertypes(): Collection<ConeKotlinType> {
         if (this is ErrorTypeConstructor) return emptyList()